Duplicate PRs created by concurrent webhook + cron triggers

[zacharias-ona]

Feature Builder and Bug Fixer automations have two triggers that can fire near-simultaneously: a PULL_REQUEST_EVENT_MERGED webhook and a */10 * * * * cron. When a PR merges close to a cron tick, both triggers spawn separate agent instances, producing duplicate PRs for the same issue.

Observed duplicates:

• Issue bug: UI regression after PR #34 — Health endpoint reports database down #36 → PRs fix: health endpoint crashes when Supabase env vars are missing (#36) #40 and fix: health endpoint crashes when Supabase env vars are missing (#36) #41 (79 seconds apart)
• Issue Workspace CRUD and workspace switcher #26 → PRs feat: workspace CRUD and workspace switcher (#26) #45 and feat: workspace CRUD and workspace switcher (#26) #46 (28 seconds apart)

Fix: Remove the webhook trigger. The cron is sufficient — work is picked up within 10 minutes.