chore: consolidate plans into .claude/plans/, archive completed plans, clean up old commands and plan dirs

- Move 22 completed plans from docs/plans/ to .claude/plans/_archive/
- Remove obsolete .claude/commands/ (loop-gap, meta-loop-gap, meta-planner)
- Remove completed plan subdirs (scrivener-import/, tree-context-view/)
- Add active plans (refactor-extension-ts, extension-hardening, rename, agent-integration-stub)
- Source code updates from recent hardening and refactoring work

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: ansonphong