[TheGreatAxios]

Summary

• Vendored Interchange HEAD rejects type: "schedule" triggers at defineWorkflow and at the hub probe gate. @corbits/cron will deliver a tick mail on each schedule instead, so every workflow that used to declare a schedule trigger becomes mail-triggered.
• heartbeat was already mail-triggered ({ type: "mail", to: input.triggerAddress }) — no change needed there.
• workbench-digest and credential-expiry now take a triggerAddress input and declare { type: "mail", to: input.triggerAddress }, following the same convention as echo/heartbeat/code-review. Removed the now-unused WORKBENCH_DIGEST_SCHEDULE_CRON / CREDENTIAL_EXPIRY_SCHEDULE_CRON exports.
• Updated the one caller of buildWorkbenchDigestWorkflow (packages/onboarding/src/tenant-seed.ts, catalog workflow entry) and the digest e2e script to pass a triggerAddress; credential-expiry has no wiring yet so nothing else to update.
• Deleted the schedule-trigger assertion in workflows/workbench-digest/test/definition.test.ts and replaced it with a mail-trigger assertion (mirroring heartbeat's), plus an empty-triggerAddress rejection test.

Test plan

• bun test workflows/heartbeat workflows/workbench-digest workflows/credential-expiry — 27 pass
• bun test packages/onboarding — 212 pass, 12 DB-gated skips (no local DATABASE_URL, expected)
• bun run typecheck && bun run lint && bun run fmt — clean
• bun run check:structural — all checks ok (no version bump required by tool-package-freshness)

[linear-code]

CL-8183