feat(spec): add XDG global config directory and config command proposals

Create two OpenSpec change proposals:
1. add-global-config-dir: Foundation for user-level configuration following XDG Base Directory Specification with cross-platform support
2. add-config-command: User-facing CLI command for viewing and managing global settings

Both proposals are minimal and focused on providing a clean, extensible base for OpenSpec settings and future feature flags.

openspec/changes/add-config-command/proposal.md

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+## Why
+
+Users need a way to view and modify their global OpenSpec settings without manually editing JSON files. The `add-global-config-dir` change provides the foundation, but there's no user-facing interface to interact with the config. A dedicated `openspec config` command provides discoverability and ease of use.
+
+## What Changes
+
+Add `openspec config` subcommand with the following operations:
+
+```bash
+openspec config path                    # Show config file location
+openspec config list                    # Show all current settings
+openspec config get <key>               # Get a specific value
+openspec config set <key> <value>       # Set a value
+openspec config reset [key]             # Reset to defaults (all or specific key)
+```
+
+**Example usage:**
+```bash
+$ openspec config path
+/Users/me/.config/openspec/config.json
+
+$ openspec config list
+enableTelemetry: true
+featureFlags: {}
+
+$ openspec config set enableTelemetry false
+Set enableTelemetry = false
+
+$ openspec config get enableTelemetry
+false
+```
+
+## Impact
+
+- Affected specs: New `cli-config` capability
+- Affected code:
+  - New `src/commands/config.ts`
+  - Update CLI entry point to register config command
+- Dependencies: Requires `add-global-config-dir` to be implemented first

openspec/changes/add-global-config-dir/design.md

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+## Context
+
+OpenSpec needs a standard location for user-level configuration that works across platforms and follows established conventions. This will serve as the foundation for settings, feature flags, and future artifacts like workflows or templates.
+
+## Goals / Non-Goals
+
+**Goals:**
+- Provide a single, well-defined location for global config
+- Follow XDG Base Directory Specification (widely adopted by CLI tools)
+- Support cross-platform usage (Unix, macOS, Windows)
+- Keep implementation minimal - just the foundation
+- Enable future expansion (cache, state, workflows)
+
+**Non-Goals:**
+- Project-local config override (not in scope)
+- Config file migration tooling
+- Config validation CLI commands
+- Multiple config profiles
+
+## Decisions
+
+### Path Resolution Strategy
+
+**Decision:** Use XDG Base Directory Specification with platform fallbacks.
+
+```
+Unix/macOS: $XDG_CONFIG_HOME/openspec/ or ~/.config/openspec/
+Windows:    %APPDATA%/openspec/
+```
+
+**Rationale:**
+- XDG is the de facto standard for CLI tools (used by gh, bat, ripgrep, etc.)
+- Environment variable override allows user customization
+- Windows uses its native convention (%APPDATA%) for better integration
+
+**Alternatives considered:**
+- `~/.openspec/` - Simple but clutters home directory
+- `~/Library/Application Support/` on macOS - Overkill for a CLI tool
+
+### Config File Format
+
+**Decision:** JSON (`config.json`)
+
+**Rationale:**
+- Native Node.js support (no dependencies)
+- Human-readable and editable
+- Type-safe with TypeScript
+- Matches project.md's "minimal dependencies" principle
+
+**Alternatives considered:**
+- YAML - Requires dependency, more error-prone to edit
+- TOML - Less common in Node.js ecosystem
+- Environment variables only - Too limited for structured settings
+
+### Config Schema
+
+**Decision:** Flat structure with typed fields, start minimal.
+
+```typescript
+interface GlobalConfig {
+  featureFlags?: Record<string, boolean>;
+}
+```
+
+**Rationale:**
+- `featureFlags` enables controlled rollout of new features
+- Optional fields with defaults avoid breaking changes
+- Flat structure is easy to understand and extend
+
+### Loading Strategy
+
+**Decision:** Read from disk on each call, no caching.
+
+```typescript
+export function getGlobalConfig(): GlobalConfig {
+  return loadConfigFromDisk();
+}
+```
+
+**Rationale:**
+- CLI commands are short-lived; caching adds complexity without benefit
+- Reading a small JSON file is ~1ms; negligible overhead
+- Always returns fresh data; no cache invalidation concerns
+- Simpler implementation
+
+### Directory Creation
+
+**Decision:** Create directory only when saving, not when reading.
+
+**Rationale:**
+- Don't create empty directories on read operations
+- Users who never save config won't have unnecessary directories
+- Aligns with principle of least surprise
+
+## Risks / Trade-offs
+
+| Risk | Mitigation |
+|------|------------|
+| Config file corruption | Return defaults on parse error, log warning |
+| Permissions issues | Check write permissions before save, clear error message |
+| Future schema changes | Use optional fields, add version field if needed later |
+
+## Open Questions
+
+None - this proposal is intentionally minimal.

openspec/changes/add-global-config-dir/proposal.md

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+## Why
+
+OpenSpec currently has no mechanism for user-level global settings or feature flags. As the CLI grows, we need a standard location to store user preferences, experimental features, and other configuration that persists across projects. Following XDG Base Directory Specification provides a well-understood, cross-platform approach.
+
+## What Changes
+
+- Add new `src/core/global-config.ts` module with:
+  - Path resolution following XDG Base Directory spec (`$XDG_CONFIG_HOME/openspec/` or fallback)
+  - Cross-platform support (Unix, macOS, Windows)
+  - Lazy config loading with sensible defaults
+  - TypeScript types for config shape
+- Export a global config directory path getter for future use (workflows, templates, cache)
+- Initial config schema supports 1-2 settings/feature flags only
+
+## Impact
+
+- Affected specs: New `global-config` capability (no existing specs modified)
+- Affected code:
+  - New `src/core/global-config.ts`
+  - Update `src/core/index.ts` to export new module

openspec/changes/add-global-config-dir/specs/global-config/spec.md

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+## ADDED Requirements
+
+### Requirement: Global Config Directory Path
+
+The system SHALL resolve the global configuration directory path following XDG Base Directory Specification with platform-specific fallbacks.
+
+#### Scenario: Unix/macOS with XDG_CONFIG_HOME set
+- **WHEN** `$XDG_CONFIG_HOME` environment variable is set to `/custom/config`
+- **THEN** `getGlobalConfigDir()` returns `/custom/config/openspec`
+
+#### Scenario: Unix/macOS without XDG_CONFIG_HOME
+- **WHEN** `$XDG_CONFIG_HOME` environment variable is not set
+- **AND** the platform is Unix or macOS
+- **THEN** `getGlobalConfigDir()` returns `~/.config/openspec` (expanded to absolute path)
+
+#### Scenario: Windows platform
+- **WHEN** the platform is Windows
+- **AND** `%APPDATA%` is set to `C:\Users\User\AppData\Roaming`
+- **THEN** `getGlobalConfigDir()` returns `C:\Users\User\AppData\Roaming\openspec`
+
+### Requirement: Global Config Loading
+
+The system SHALL load global configuration from the config directory with sensible defaults when the config file does not exist or cannot be parsed.
+
+#### Scenario: Config file exists and is valid
+- **WHEN** `config.json` exists in the global config directory
+- **AND** the file contains valid JSON matching the config schema
+- **THEN** `getGlobalConfig()` returns the parsed configuration
+
+#### Scenario: Config file does not exist
+- **WHEN** `config.json` does not exist in the global config directory
+- **THEN** `getGlobalConfig()` returns the default configuration
+- **AND** no directory or file is created
+
+#### Scenario: Config file is invalid JSON
+- **WHEN** `config.json` exists but contains invalid JSON
+- **THEN** `getGlobalConfig()` returns the default configuration
+- **AND** a warning is logged to stderr
+
+### Requirement: Global Config Saving
+
+The system SHALL save global configuration to the config directory, creating the directory if it does not exist.
+
+#### Scenario: Save config to new directory
+- **WHEN** `saveGlobalConfig(config)` is called
+- **AND** the global config directory does not exist
+- **THEN** the directory is created
+- **AND** `config.json` is written with the provided configuration
+
+#### Scenario: Save config to existing directory
+- **WHEN** `saveGlobalConfig(config)` is called
+- **AND** the global config directory already exists
+- **THEN** `config.json` is written (overwriting if exists)
+
+### Requirement: Default Configuration
+
+The system SHALL provide a default configuration that is used when no config file exists.
+
+#### Scenario: Default config structure
+- **WHEN** no config file exists
+- **THEN** the default configuration includes an empty `featureFlags` object
+
+### Requirement: Config Schema Evolution
+
+The system SHALL merge loaded configuration with default values to ensure new config fields are available even when loading older config files.
+
+#### Scenario: Config file missing new fields
+- **WHEN** `config.json` exists with `{ "featureFlags": {} }`
+- **AND** the current schema includes a new field `defaultAiTool`
+- **THEN** `getGlobalConfig()` returns `{ featureFlags: {}, defaultAiTool: <default> }`
+- **AND** the loaded values take precedence over defaults for fields that exist in both
+
+#### Scenario: Config file has extra unknown fields
+- **WHEN** `config.json` contains fields not in the current schema
+- **THEN** the unknown fields are preserved in the returned configuration
+- **AND** no error or warning is raised

openspec/changes/add-global-config-dir/tasks.md

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+## 1. Core Implementation
+
+- [ ] 1.1 Create `src/core/global-config.ts` with path resolution
+  - Implement `getGlobalConfigDir()` following XDG spec
+  - Support `$XDG_CONFIG_HOME` environment variable override
+  - Platform-specific fallbacks (Unix: `~/.config/`, Windows: `%APPDATA%`)
+- [ ] 1.2 Define TypeScript interfaces for config shape
+  - `GlobalConfig` interface with optional fields
+  - Start minimal: just `featureFlags?: Record<string, boolean>`
+- [ ] 1.3 Implement config loading with defaults
+  - `getGlobalConfig()` - reads config.json if exists, merges with defaults
+  - No directory/file creation on read (lazy initialization)
+- [ ] 1.4 Implement config saving
+  - `saveGlobalConfig(config)` - writes config.json, creates directory if needed
+
+## 2. Integration
+
+- [ ] 2.1 Export new module from `src/core/index.ts`
+- [ ] 2.2 Add constants for config file name and directory name
+
+## 3. Testing
+
+- [ ] 3.1 Manual testing of path resolution on current platform
+- [ ] 3.2 Test with/without `$XDG_CONFIG_HOME` set
+- [ ] 3.3 Test config load when file doesn't exist (should return defaults)