## Summary
Adds a single env flag, `DEPRECATE_V3_ENABLED` (default off), that
gracefully winds down the v3 engine (`RunEngineVersion.V1`). While it's
off nothing changes, so self-hosted instances still on v3 keep working.
When it's on:
- Triggers that resolve to v3 are rejected with a clear, actionable
error pointing at the [v4 migration
guide](https://trigger.dev/docs/migrating-from-v3), instead of silently
creating runs that never execute. This covers single triggers, batches,
scheduled fires, replays, and `triggerAndWait`, which all funnel through
one place.
- The legacy `trigger dev` websocket used by v3 CLIs is closed with an
upgrade message (v4 CLIs use a different dev transport).
- The v3 shared-queue consumer refuses to start, so no deployed v3 runs
are dequeued.
- The v3 run-lifecycle background jobs (heartbeat timeout, TTL expiry,
retry, resume batch/dependency, delayed-run enqueue, and scheduled
fires) become no-ops, so abandoned v3 runs stop generating database
load.
This builds on the existing deploy deprecation flag, which already
rejects v3 CLI deploys.
## Design
Enforcement is read through one helper, `isV3Disabled()`. Every gate
combines it with a per-run or per-project engine check (`isV3Disabled()
&& engine === "V1"`), so a v4 run that happens to reach a shared service
behaves exactly as before. v4 (V2) is never affected.
The flag is a hard switch, not a drain: when it's on, in-flight v3 runs
are abandoned in place rather than failed or expired, which is the
intended behaviour for the final shutdown.
Eric Allam
f1bd11a7ef