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/collaboration/git-sync-converter-gate.spec.ts

@@ -16,7 +16,7 @@
  * editor-ext ProseMirror documents must survive a full round trip through the
  * actual server write path without losing any node / mark / attribute.
  *
- * Pipeline per document (plan §13.1):
+ * Pipeline per document (issue #194 §13.1):
  *   1. md   = convertProseMirrorToMarkdown(content)          // git-sync export
  *   2. doc  = await markdownToProseMirror(md)                // git-sync import
  *   3. push `doc` through the REAL editor-ext Yjs write path the server uses:
@@ -26,12 +26,12 @@
  *      (apps/server/src/collaboration/extensions/persistence.extension.ts:96/115)
  *      with the same `tiptapExtensions` (collaboration.util.ts) and the same
  *      `@hocuspocus/transformer`, so the gate exercises the real schema
- *      validation that runs on a git-sync write (plan §3.3).
+ *      validation that runs on a git-sync write (issue #194 §3.3).
  *   4. assert docsCanonicallyEqual(canon(original), canon(normalized)) === true
  *
  * Any node / mark / attr that editor-ext drops (because the git-sync
  * docmost-schema named it differently, or declares a different default) makes
- * the gate FAIL for that document — exactly the schema-divergence plan §3.3 /
+ * the gate FAIL for that document — exactly the schema-divergence issue #194 §3.3 /
  * §13.1 warn about. Genuine, irreducible divergences are isolated into the
  * clearly-named `KNOWN DIVERGENCE` block at the bottom (never silently hidden).
  *