refactor(cron): rename scheduler feature to cron

Rename the recurring-jobs feature scheduler->cron to disambiguate from the
legacy ScheduledRun system (/api/scheduled), which is left untouched:

- ScheduledJob->CronJob, SchedulerService->CronService
- /api/scheduler/jobs -> /api/cron/jobs; SSE scheduler:* -> cron:*
- state keys cronJobs/cronJobRuns
- files moved to src/cron/, cron-routes.ts, cron-port.ts, types/cron.ts
- frontend cron-ui.js, #cronModal, menu "Cron"
- docs moved to docs/cron-discovery.md + docs/cron-build-brief.md, README guides
- new tests: cron-service.test.ts, cron-time.test.ts

Green: tsc, lint, frontend-syntax, format, 30 cron + 9 legacy scheduled-runs tests.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PmvZR12aX2v8K7YhqxPUAU

src/web/public/constants.js

@@ -276,11 +276,11 @@ const SSE_EVENTS = {
   SCHEDULED_LOG: 'scheduled:log',
   SCHEDULED_DELETED: 'scheduled:deleted',
 
-  // Scheduled jobs (cron-style scheduler)
-  SCHEDULER_JOBS_CHANGED: 'scheduler:jobsChanged',
-  SCHEDULER_JOB_DELETED: 'scheduler:jobDeleted',
-  SCHEDULER_RUN_CREATED: 'scheduler:runCreated',
-  SCHEDULER_RUN_UPDATED: 'scheduler:runUpdated',
+  // Cron jobs
+  CRON_JOBS_CHANGED: 'cron:jobsChanged',
+  CRON_JOB_DELETED: 'cron:jobDeleted',
+  CRON_RUN_CREATED: 'cron:runCreated',
+  CRON_RUN_UPDATED: 'cron:runUpdated',
 
   // Respawn
   RESPAWN_STARTED: 'respawn:started',