Move the engineering specs out of plans/ into a root docs/ folder.
architecture/ describes what is true in the code now, one doc per
feature. adr/ records decisions: 0003 is restored and 0004-0027 are
new. proposals/ holds designs not built yet: plugins and the CUDA
backend. contracts/ holds the frozen interfaces between trees, with
the fixtures their tests read. ROADMAP.md names the initiatives.
The shipped phase plans are deleted; git history keeps them. plans/
keeps business, launch and ops material only.
Also: CONTRIBUTING.md rewritten around this flow, a CODEOWNERS file,
a Docs section in AGENTS.md, scripts/check_docs.py (was
check_plan_links.py) as a pre-commit hook, the backend tests path
filter extended to docs/contracts, and the five tests that read the
contract fixtures pointed at their new path.
Co-authored-by: Cursor <cursoragent@cursor.com>
