workflow: add_reaction action calls a nonexistent relay endpoint — any workflow using it always fails

[leesalminen]

Summary

The workflow engine's add_reaction action can never succeed: it POSTs to {BUZZ_RELAY_BASE_URL:-http://localhost:3000}/api/messages/{message_id}/reactions (crates/buzz-workflow/src/executor.rs, add_reaction_impl), but the relay has no such route — router.rs defines no /api/messages/* paths. Every invocation gets the SPA/404 fallback:

webhook error: AddReaction: relay returned 404 Not Found for message <id>:

Because a failed step aborts the run, any workflow that uses add_reaction as an early step (e.g. an ack-reaction before a send_message) silently kills the whole workflow — from the user's perspective, "the workflow doesn't fire."

Repro

1. Create a message_posted workflow whose first step is action: add_reaction.
2. Post a matching message.
3. workflow_runs shows status=failed, error_message as above; subsequent steps never run.
4. curl -X POST http://127.0.0.1:3000/api/messages/deadbeef/reactions → 404 (route absent).

Also note the manual-trigger path (kind:46020) fails differently — "AddReaction: no trigger.message_id available" — which is arguably correct, but combined with the missing route the action is unusable everywhere.

Suggested fix

Either implement the action through the relay's real reaction path (build a kind:7 event via the workflow sink, like send_message does — no HTTP round-trip needed since the engine runs in-process), or remove/feature-gate the action until it works. If kept HTTP-based, it also needs the host-scoped community binding (localhost:3000 vs the deployment host) taken into account.

Found while debugging with #2385 / #2390 — happy to PR the in-process kind:7 approach.