v0.6.0 - Unified Session Tracking and Boot Context Auto-Injection

Features:
- Plugin auto-injects boot context on first message of each session
- Boot gates cascade by directory hierarchy (project → parent dir → global)
- Session registration tracks boot context IDs for de-duplication
- Enhanced memory usage instructions embedded in boot context
- Include conversation_summary in date-range queries for session history

Changes:
- Setup script offers to clean up legacy AGENTS.md (no longer needed)
- README updated to reflect auto-injection behavior
- Add _memory_ids_shown and _token_estimate to boot context response