feat: add OpenSpec change proposal for archive command

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

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+## Why
+Need a command to archive completed changes to the archive folder with proper date prefixing, following OpenSpec conventions. Currently changes must be manually moved and renamed.
+
+## What Changes
+- Add new `archive` command to CLI that moves changes to `changes/archive/YYYY-MM-DD-[change-name]/`
+- Check for incomplete tasks before archiving and warn user
+- Allow interactive selection of change to archive
+- Prevent archiving if target directory already exists
+
+## Impact
+- Affected specs: cli-archive (new)
+- Affected code: src/cli/index.ts, src/core/archive.ts (new)

openspec/changes/add-archive-command/specs/cli-archive/spec.md

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+# CLI Archive Command Specification
+
+## Purpose
+The archive command moves completed changes from the active changes directory to the archive folder with date-based naming, following OpenSpec conventions.
+
+## Command Syntax
+```bash
+openspec archive [change-name]
+```
+
+## Behavior
+
+### Change Selection
+WHEN no change-name is provided
+THEN display interactive list of available changes (excluding archive/)
+AND allow user to select one
+
+WHEN change-name is provided
+THEN use that change directly
+AND validate it exists
+
+### Task Completion Check
+The command SHALL scan the change's tasks.md file for incomplete tasks (marked with `- [ ]`)
+
+WHEN incomplete tasks are found
+THEN display all incomplete tasks to the user
+AND prompt for confirmation to continue
+AND default to "No" for safety
+
+WHEN all tasks are complete OR no tasks.md exists
+THEN proceed with archiving without prompting
+
+### Archive Process
+The archive operation SHALL:
+1. Create archive/ directory if it doesn't exist
+2. Generate target name as `YYYY-MM-DD-[change-name]` using current date
+3. Check if target directory already exists
+4. Move the entire change directory to the archive location
+
+WHEN target archive already exists
+THEN fail with error message
+AND do not overwrite existing archive
+
+WHEN move succeeds
+THEN display success message with archived name
+
+## Error Handling
+
+SHALL handle the following error conditions:
+- Missing openspec/changes/ directory
+- Change not found
+- Archive target already exists
+- File system permissions issues
+
+## Why These Decisions
+
+**Interactive selection**: Reduces typing and helps users see available changes
+**Task checking**: Prevents accidental archiving of incomplete work
+**Date prefixing**: Maintains chronological order and prevents naming conflicts
+**No overwrite**: Preserves historical archives and prevents data loss

openspec/changes/add-archive-command/tasks.md

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+# Implementation Tasks
+
+## 1. Core Implementation
+- [ ] 1.1 Create `src/core/archive.ts` with ArchiveCommand class
+  - [ ] 1.1.1 Implement change selection (interactive if not provided)
+  - [ ] 1.1.2 Implement incomplete task checking from tasks.md
+  - [ ] 1.1.3 Implement confirmation prompt for incomplete tasks
+  - [ ] 1.1.4 Implement archive move with date prefixing
+
+## 2. CLI Integration
+- [ ] 2.1 Add archive command to `src/cli/index.ts`
+  - [ ] 2.1.1 Import ArchiveCommand
+  - [ ] 2.1.2 Register command with commander
+  - [ ] 2.1.3 Add proper error handling
+
+## 3. Error Handling
+- [ ] 3.1 Handle missing openspec/changes/ directory
+- [ ] 3.2 Handle change not found
+- [ ] 3.3 Handle archive target already exists
+- [ ] 3.4 Handle user cancellation
+
+## 4. Testing
+- [ ] 4.1 Test with fully completed change
+- [ ] 4.2 Test with incomplete tasks (warning shown)
+- [ ] 4.3 Test interactive selection mode
+- [ ] 4.4 Test duplicate archive prevention
+
+## 5. Build and Validation
+- [ ] 5.1 Ensure TypeScript compilation succeeds
+- [ ] 5.2 Test command execution