docs: Archive outdated PLAN.md as historical reference

PLAN.md was written for initial MVP development and is now outdated.
Project has progressed well beyond original scope:

- v0.3.0 released (all Phase 1-2 features complete)
- Production concurrency utilities added
- Comprehensive test suite (294 tests)
- Full documentation suite complete
- Published to npm

Archived to docs/archive/PLAN-initial.md for historical reference.

🤖 Generated with Claude Code

Co-Authored-By: Claude <[email protected]>

Author: michaeloboyle