docs(git-sync): document GIT_SYNC_BACKEND_TIMEOUT_MS, drop dead consts, fix dangling plan refs

Address the non-red-team documentation/cleanup items from review #1679:
- Document the GIT_SYNC_BACKEND_TIMEOUT_MS watchdog (git http-backend) in
  .env.example and add it to the environment validation schema — it was used
  (getGitSyncBackendTimeoutMs, default 120000) but undocumented/unvalidated.
- Remove the dead GIT_SYNC_DEBOUNCE_MS_DEFAULT / GIT_SYNC_POLL_INTERVAL_MS_DEFAULT
  exports (never imported; environment.service is the single source of defaults).
- Redirect the dangling `plan §X.Y` comment references to issue #194 (the
  git-sync spec moved there when docs/git-sync-plan.md was deleted by this PR).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

apps/server/src/integrations/environment/environment.service.ts

@@ -333,7 +333,7 @@ export class EnvironmentService {
       .filter(Boolean);
   }
 
-  // --- git-sync (plan §7.2) -------------------------------------------------
+  // --- git-sync (issue #194 §7.2) -------------------------------------------------
 
   /** Global master switch for the git-sync control plane (default false). */
   isGitSyncEnabled(): boolean {