feat: add OpenSpec slash commands for Claude Code

- Remove .claude/ from .gitignore to share commands with team
- Add /openspec:proposal command for creating change proposals
- Add /openspec:apply command for implementing changes
- Add /openspec:archive command for archiving completed changes

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

.gitignore

@@ -73,5 +73,4 @@ _testing-suite/
 # Claude Code tools
 .playwright-mcp/
 .serena/
-.claude/
 nul