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/common/decorators/auth-provenance.decorator.ts

@@ -10,7 +10,7 @@ import { ProvenanceSource } from '../../core/auth/dto/jwt-payload';
  */
 export interface AuthProvenanceData {
   // ProvenanceSource includes 'git-sync' — set by the in-process git-sync data
-  // plane (plan §8.1) when it drives PageService writes; never from a request token.
+  // plane (issue #194 §8.1) when it drives PageService writes; never from a request token.
   actor: ProvenanceSource;
   aiChatId: string | null;
 }
@@ -62,7 +62,7 @@ export function agentSourceFields<S extends string, C extends string>(
   sourceKey: S,
   chatKey: C,
 ): Partial<Record<S, ProvenanceSource> & Record<C, string | null>> {
-  // git-sync data-plane write (plan §8.1): stamp the source 'git-sync' with NO
+  // git-sync data-plane write (issue #194 §8.1): stamp the source 'git-sync' with NO
   // aiChatId (it has no internal ai_chats row). Mirrors the agent branch; each
   // write has a single actor, so precedence is irrelevant here.
   if (provenance?.actor === 'git-sync') {