feat: add background agents with scheduling support

- Add background task scheduling system with cron-based triggers
- Add background-task-detail component for viewing agent status
- Add agent schedule repo and state management
- Update sidebar to show background agents section
- Remove old workflow-authoring and workflow-run-ops skills
- Add IPC handlers for agent schedule operations

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

apps/x/packages/shared/src/agent-schedule-state.ts

@@ -0,0 +1,17 @@
+import z from "zod";
+
+// "triggered" is terminal state for once-schedules (will not run again)
+export const AgentScheduleStatus = z.enum(["scheduled", "running", "finished", "failed", "triggered"]);
+
+export const AgentScheduleStateEntry = z.object({
+    status: AgentScheduleStatus,
+    startedAt: z.string().nullable(), // When current run started (for timeout detection)
+    lastRunAt: z.string().nullable(), // ISO 8601 local datetime
+    nextRunAt: z.string().nullable(), // ISO 8601 local datetime
+    lastError: z.string().nullable(),
+    runCount: z.number().default(0),
+});
+
+export const AgentScheduleState = z.object({
+    agents: z.record(z.string(), AgentScheduleStateEntry),
+});