[codex] Guide watcher jobs to avoid unchanged poll events

[zzj3720]

Summary

• clarify that BROKER_JOB_HELPER event is for material async changes only
• tell watcher jobs to heartbeat or stay silent when CI/PR state is unchanged
• change the generated watcher registration example so it no longer demonstrates emitting an event as a polling tick

Validation

• pnpm exec vitest run test/app-server-client.test.ts test/slack-message-format.test.ts
• pnpm exec tsc -p tsconfig.json --noEmit
• pnpm build
• pnpm test