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/types/app-state.ts

@@ -23,7 +23,7 @@ import type { SessionState } from './session.js';
 import type { TaskState } from './task.js';
 import type { RalphLoopState } from './ralph.js';
 import type { RespawnConfig } from './respawn.js';
-import type { ScheduledJob, ScheduledJobRun } from './scheduler.js';
+import type { CronJob, CronJobRun } from './cron.js';
 
 // ========== Global Stats Types ==========
 
@@ -113,9 +113,9 @@ export interface AppState {
   /** Orchestrator Loop state (phased plan execution) */
   orchestrator?: import('./orchestrator.js').OrchestratorPersistState;
   /** Cron-style scheduled jobs, keyed by job ID. */
-  scheduledJobs?: Record<string, ScheduledJob>;
+  cronJobs?: Record<string, CronJob>;
   /** Scheduled job run history, keyed by run ID. */
-  scheduledJobRuns?: Record<string, ScheduledJobRun>;
+  cronJobRuns?: Record<string, CronJobRun>;
 }
 
 // ========== Default Configuration ==========