chore: archive completed spec and plan files to docs/archive

Move to docs/archive/:
- plan.md (TDD implementation plan — all phases complete)
- phase11-mcp-tools-spec.md (MCP tool expansion spec)
- phase12-language-expansion-spec.md (language expansion spec)
- phase-age-spec.md (Apache AGE integration spec)
- PYTHON_PROJECT_FEATURE_INVENTORY.md (reference project analysis)

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: tae2089