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/cron/cron-input.ts

@@ -0,0 +1,30 @@
+/**
+ * @fileoverview Input shape for creating/updating a cron job. This is the
+ * user-settable subset of `CronJob` (server-maintained bookkeeping fields
+ * such as nextRunAt / lastStatus are excluded). Produced by the zod schema.
+ */
+
+import type { ConcurrencyPolicy, InputMode, PromptMode, ScheduleType } from '../types/cron.js';
+import type { SessionMode } from '../types/session.js';
+
+export type { CronJob, CronJobRun, CronJobRunStatus, TriggerType } from '../types/cron.js';
+
+export interface CronJobInput {
+  name: string;
+  agentType: SessionMode;
+  workingDir: string;
+  launchCommand?: string;
+  promptMode: PromptMode;
+  promptText?: string;
+  promptFilePath?: string;
+  inputMode: InputMode;
+  scheduleType: ScheduleType;
+  runAt?: number;
+  intervalMinutes?: number;
+  dailyTime?: string;
+  weeklyDays?: number[];
+  weeklyTime?: string;
+  enabled: boolean;
+  notes?: string;
+  concurrencyPolicy: ConcurrencyPolicy;
+}