Add changeset for OPSX experimental workflow commands (#457)

.changeset/opsx-experimental-workflow.md

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+---
+"@fission-ai/openspec": minor
+---
+
+Add OPSX experimental workflow commands and enhanced artifact system
+
+**New Commands:**
+- `/opsx:ff` - Fast-forward through artifact creation, generating all needed artifacts in one go
+- `/opsx:sync` - Sync delta specs from a change to main specs
+- `/opsx:archive` - Archive completed changes with smart sync check
+
+**Artifact Workflow Enhancements:**
+- Schema-aware apply instructions with inline guidance and XML output
+- Agent schema selection for experimental artifact workflow
+- Per-change schema metadata via `.openspec.yaml` files
+- Agent Skills for experimental artifact workflow
+- Instruction loader for template loading and change context
+- Restructured schemas as directories with templates
+
+**Improvements:**
+- Enhanced list command with last modified timestamps and sorting
+- Change creation utilities for better workflow support
+
+**Fixes:**
+- Normalize paths for cross-platform glob compatibility
+- Allow REMOVED requirements when creating new spec files