feat: add persistent cron-based recurring reminders

Replace in-memory-only setTimeout scheduling with a DB-backed system
that survives broker restarts. Adds:

- `scheduled_message` table in mesh schema (Drizzle + raw CREATE TABLE
  for zero-downtime deploys)
- Minimal 5-field cron parser (no dependencies) with next-fire-time
  calculation for recurring entries
- On broker boot, all non-cancelled entries are loaded from PostgreSQL
  and timers re-armed automatically
- CLI `schedule_reminder` MCP tool accepts optional `cron` expression
- CLI `remind` command accepts `--cron` flag
- One-shot reminders remain backward compatible — no cron field = same
  behavior as before

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/cli/src/mcp/server.ts

@@ -463,16 +463,22 @@ Your message mode is "${messageMode}".
           to?: string;
           deliver_at?: number;
           in_seconds?: number;
+          cron?: string;
         };
         if (!sArgs.message) return text("schedule_reminder: `message` required", true);
 
+        const isCron = !!sArgs.cron;
+
         let deliverAt: number;
-        if (sArgs.deliver_at) {
+        if (isCron) {
+          // For cron, deliverAt is ignored by the broker — set to 0
+          deliverAt = 0;
+        } else if (sArgs.deliver_at) {
           deliverAt = Number(sArgs.deliver_at);
         } else if (sArgs.in_seconds) {
           deliverAt = Date.now() + Number(sArgs.in_seconds) * 1_000;
         } else {
-          return text("schedule_reminder: provide `deliver_at` (ms timestamp) or `in_seconds`", true);
+          return text("schedule_reminder: provide `deliver_at` (ms timestamp), `in_seconds`, or `cron` expression", true);
         }
 
         const isSelf = !sArgs.to;
@@ -496,8 +502,18 @@ Your message mode is "${messageMode}".
           }
         }
 
-        const result = await client.scheduleMessage(targetSpec, sArgs.message, deliverAt, true);
+        const result = await client.scheduleMessage(targetSpec, sArgs.message, deliverAt, true, sArgs.cron);
         if (!result) return text("schedule_reminder: broker did not acknowledge — check connection", true);
+
+        if (isCron) {
+          const nextFire = new Date(result.deliverAt).toISOString();
+          return text(
+            isSelf
+              ? `Recurring self-reminder scheduled (${result.scheduledId.slice(0, 8)}): "${sArgs.message.slice(0, 60)}" — cron: ${sArgs.cron}, next fire: ${nextFire}`
+              : `Recurring reminder to "${sArgs.to}" scheduled (${result.scheduledId.slice(0, 8)}) — cron: ${sArgs.cron}, next fire: ${nextFire}`,
+          );
+        }
+
         const when = new Date(result.deliverAt).toISOString();
         return text(
           isSelf