Parity audit columns + wiring (source+head_sha; computeGateParity ported)

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Part of #983.

Context

The cutover safety model depends on computeGateParity (computeGateParity/isParityCutoverReady, ALREADY ported as src/review/parity; MIN_PARITY_SAMPLE=30, agreement floor ~0.98, ZERO unsafe disagreements) which JOINs latest gate-decision rows per (project, target, head_sha) across two source writers in a comparable audit store. gittensory's audit store is auditEvents (db/repositories.ts recordAuditEvent; schema columns id/eventType/actor/route/targetKey/outcome/detail/metadataJson/createdAt) with NO source and NO head_sha (head_sha exists on check_summaries/recommendation_snapshots, not on auditEvents). So parity CANNOT run as-is. This issue adds the audit columns + wires the ported parity module so shadow-parity can run; #1013-era reviewbot rows are tagged source=reviewbot, gittensory rows source=gittensory (migration 0019 is the reference).

Deliverables

• Add source + head_sha columns to a gittensory gate-decision audit row (new migration; reference reviewbot migration 0019).
• Tag every gate decision with its writer source (gittensory vs reviewbot) and the PR head_sha on write.
• Wire the ported src/review/parity (computeGateParity/isParityCutoverReady) over the source-tagged audit rows so shadow-parity can run (floor ~0.98, min 30, zero unsafe disagreements).
• Reconcile collecting both writers' decisions into one analysis store keyed by (repo, pr, head_sha) — cross-account D1 is impractical, so export/import into one store.
• Add tests: parity computes agreement + unsafe-disagreement count over source-tagged rows; isParityCutoverReady gates on floor + sample.

Acceptance criteria

• A gate-decision audit row carries source + head_sha; both writers' decisions are comparable in one store keyed by (repo, pr, head_sha).
• computeGateParity runs over the source-tagged rows and isParityCutoverReady returns ready only at ≥30 paired samples, ≥ floor agreement, zero unsafe disagreements.

Notes

• Milestone: M1
• Depends on: Wire ops/stats/alerts/self-improve (modules ported) + cron tick + tunables D1 #1022
• computeGateParity is ALREADY ported (src/review/parity); scope here is the audit columns + wiring, not re-porting. This is a hard prerequisite for a safe per-repo flip (feat(agent): return useful decision packs while snapshots refresh #15) — without it the cutover loses its primary safety net.

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Closing as completed (2026-07-01 issue audit): Parity audit columns (migration 0049 source+head_sha) + computeGateParity wiring shipped (parity.ts/parity-wire.ts).