docs: archive stale findings docs (work completed)

Both findings docs described a codebase that no longer exists — their
headline 'Critical'/'P0' items (server.ts/app.js/types.ts splits, the
{WORKING_DIR} placeholder bug) are all resolved. Moved to docs/archive/
with dated banners so they read as history, not a live TODO.

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