Fix event ingress dropping real broker events: accept null optional fields

[willwashburn]

Bug

The broker serializes absent optional fields as null; the #214 ingress schemas used .optional() (accepts missing, rejects null), so real events were dropped at ingress — agent_spawned, worker_ready, and critically relay_inbound (chat messages). Observed live:

[broker] Dropped malformed broker event: { kind: 'relay_inbound',
  reason: 'thread_id: Expected string, received null' }

(The throttled malformed-event telemetry from #195/#214 is what surfaced this.)

Fix

All 30 optional scalar fields in src/shared/schemas/broker-events.ts → .nullish() (accepts null and undefined; still rejects wrong non-null types). The three z.unknown().optional() fields already accepted null. This restores pre-#214 behavior, where null-bearing events flowed through untouched.

Why tests missed it

Fixtures were derived from the SDK's TypeScript declaration (provider?: string) — TS optionality says nothing about JSON null on the wire. New regression tests pin the exact three dropped payloads from production logs, plus a wrong-type case proving validation still rejects garbage.

Verification

• npx vitest run src/main/broker.test.ts → 76/76 (4 new)
• npx vitest run → 349/349 · npm run typecheck ✓ both configs · npm run lint ✓ 0 errors

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

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Run ID: 8140d82c-160b-4d3f-9cec-b02b4ff1a353

📥 Commits

Reviewing files that changed from the base of the PR and between 2023bac and d15ba63.

📒 Files selected for processing (2)

• src/main/broker.test.ts
• src/shared/schemas/broker-events.ts

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/event-schema-nullable-fields

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