Adds migrations/ directory with README documenting the convention
(timestamped filenames, idempotent SQL, forward-only, single change per file).
First migration is the Stage 3 queue routing columns added live during
Phase A patches today: state_type, state_type_confidence,
supersedes_prior_state, state_type_rationale, plus index on supersedes.
Required by stage2_worker.py >= 2.2 and stage3_worker.py >= 2.3.
Idempotent (IF NOT EXISTS), safe to re-apply. Verified by re-applying
against the live DB — no changes, no errors.
Closes a reproducibility gap: a fresh DB provisioned from git would crash
on first Stage 2 enqueue without these columns. Now the SQL travels with
the code.
