[TheGreatAxios]

Summary

The vendored Interchange workflow-trigger grammar
(vendor/intx/workflow/src/definition/triggers.ts) has no schedule
trigger and no scheduler. @corbits/cron is a small experimental bridge
instead: a tenant saves a cron expression plus a mail
(to/subject/body) on its own cron.schedule table, and
createCronTicker polls for due schedules and delivers each as plain
mail through the host's transport — a schedule-triggered workflow becomes
a mail-triggered one addressed at itself, so workflows keep the plain
mail trigger they already have.

• src/cron.ts — the 5-field cron parser, moved unchanged from the now
  retired @corbits/workflow-schedule (which is deleted entirely).
• src/schema.ts — the cron.schedule table (own cron Postgres schema,
  FK'd to Interchange's native tenant table) plus its migration,
  registered in scripts/db-setup.ts alongside every other installed
  package.
• src/ticker.ts — createCronTicker({ db, deliver, intervalMs }):
  claims due rows with SELECT ... FOR UPDATE SKIP LOCKED in one
  transaction so two concurrent tickers never double-fire a row, and only
  the latest missed tick fires (a schedule down for hours fires once on
  recovery, not once per missed minute).
• src/mount.ts — mountCron(app, { db, requireTenantMember }): CRUD at
  /api/tenants/:tenantId/cron.

@corbits/workflow-schedule's only other consumer,
@corbits/workflows' scheduleCronFromProjection (native
ScheduleTrigger parsing off a frozen projection), is deleted alongside
it — nothing outside that one file read it, and native schedule triggers
don't fire against the vendored grammar anyway. packages/cron needs an
explicit ruling added to scripts/checks/no-product-tenancy.ts for its
one table, same as chat/webhook-triggers/notify/etc.

Suggested hub mount (not wired up — apps/hub/src/index.ts intentionally untouched)

const cronApp = new Hono<TenantEnv>();
mountCron(cronApp, {
  db,
  requireTenantMember: (ctx, tenantId) => {
    const c = ctx as { get(key: "tenant"): { id: string } };
    return c.get("tenant").id === tenantId;
  },
});
app.route("/", cronApp);

createCronTicker({
  db,
  intervalMs: 60_000,
  deliver: (message) =>
    lookups.persistMail({
      senderAddress: message.from,
      recipients: message.to,
      raw: buildRawMessage(message),
    }),
}).start();

Test plan

• bun run typecheck
• bun run lint
• bun run fmt
• bun run check:structural
• bun test packages/cron (unit + DB-gated, against local Postgres)
• CI (DB-gated ticker suite + full monorepo bun run check)

[linear-code]

CL-8183