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/core/auth/dto/jwt-payload.ts

@@ -4,7 +4,7 @@
  * Single source of truth so a typo like 'agnet' can't slip through as a bare
  * string (#143 review). Distinct from `ActorType` (auth principal kind).
  *
- * 'git-sync' marks writes made by the git-sync data plane (plan §8.1). It NEVER
+ * 'git-sync' marks writes made by the git-sync data plane (issue #194 §8.1). It NEVER
  * travels in a user-facing token; it is set in-process on the collab connection
  * context by the native datasource, so it cannot be spoofed from a request.
  */