docs: Add implementation plan for OpenAPI refactor

37 tasks across 12 phases (A-L), TDD-style with bite-sized steps.
Each task: failing test, run-and-verify, minimal implementation, verify
green, commit. Builds new packages alongside old, cuts over OpenApiServer
in one task, then deletes legacy code.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: thced