docs: remove sprint narrative from root files (Phase 2 L1)

docs(root): move ACCESSIBILITY and SPECS out of root to their proper sections

docs(archive): consolidate audits, migration, pre-v2 roadmap under docs/archive/

test(architecture): eliminate subprocess pytest/mypy from tests

docs: refonte Diataxis + 8 documents institutionnels (S60)

feat(sprint-A10): gouvernance institutionnelle (CODEOWNERS + GOVERNANCE + CoC + COI)