feat(gatekeeper): add PR merge automation service

docs/1-getting-started/3-configuration/1-environment.md

@@ -94,6 +94,21 @@ OIDC_REDIRECT_URI=http://localhost:3001/auth/oauth2/callback/authentik
 
 See [Authentik Setup](2-authentik.md) for complete OIDC configuration.
 
+## Webhooks and Merge Automation
+
+```bash
+# Gitea webhook validation secret for /api/gatekeeper/webhook/gitea
+GITEA_WEBHOOK_SECRET=your-random-webhook-secret
+
+# Personal access token used by Gatekeeper to comment on and merge PRs
+GITEA_API_TOKEN=your-gitea-api-token
+
+# Master switch for the Gatekeeper auto-merge workflow
+GATEKEEPER_ENABLED=true
+```
+
+Use a dedicated Gitea token with the minimum repository scope needed to comment on pull requests and perform merges.
+
 ## Cache and Storage
 
 ### Valkey (Redis-compatible)