Add cron changed plugin hook (#72773)

* feat: add cron changed plugin hook

* fix: improve cron_changed hook correctness and code quality

- Fix PluginHookGatewayCronDeliveryStatus: replace 'error' with 'unknown'
  to match internal CronDeliveryStatus enum
- Add job snapshot to CronEvent so removed events carry the deleted job
- Extract pickDefined helper, replace 14-field verbose spread mapping
- Add toPluginCronJob mapper for explicit internal→public type boundary
- Fix schedule union: use literal-only kind discriminants for TS narrowing
- Use loadConfig() (runtime) instead of params.cfg (startup) in hook ctx
- Use formatErrorMessage instead of String(err) for stack preservation
- Fix pre-existing getCron TS2322 with explicit cast (matches gateway_start)
- Re-export supporting types from hooks.ts for plugin consumers
- Add tests: removed events with job, finished with full fields, runtime cfg

docs/plugins/sdk-overview.md

@@ -300,6 +300,7 @@ semantics.
 - `message_received`: use the typed `threadId` field when you need inbound thread/topic routing. Keep `metadata` for channel-specific extras.
 - `message_sending`: use typed `replyToId` / `threadId` routing fields before falling back to channel-specific `metadata`.
 - `gateway_start`: use `ctx.config`, `ctx.workspaceDir`, and `ctx.getCron?.()` for gateway-owned startup state instead of relying on internal `gateway:startup` hooks.
+- `cron_changed`: observe gateway-owned cron lifecycle changes. Use `event.job?.state?.nextRunAtMs` and `ctx.getCron?.()` when syncing external wake schedulers, and keep OpenClaw as the source of truth for due checks and execution.
 
 ### API object fields