fix(git-sync): kill spurious marker-leaking conflict, concurrent-edit loss, flapping HEAD

Three more git-sync QA defects from the 2nd live pass on PR #119, plus a
callout-fidelity nit:

1. SPURIOUS conflict leaked raw markers into canonical main (root cause). On an
   ordinary round-trip the only difference between the docmost mirror (normalize-
   on-write) and a user's raw push is trailing/empty-line normalization, which made
   git's line-based docmost->main merge CONFLICT, and the wedge fix then committed
   the file WITH literal <<<<<<< / ======= / >>>>>>> markers onto main (git and the
   DB silently diverged for cycles). Fix: on a conflict, normalize trailing/empty
   lines on BOTH sides (showStage :2:/:3:) before comparing — a trailing-only diff
   is recognized as spurious and resolved to the clean normalized form. A GENUINE
   same-block conflict is auto-resolved to OURS (git wins, mirroring the live-doc
   3-way rule); the docmost side stays on the `docmost` branch + page history. Raw
   markers NEVER reach main again.

2. Concurrent UI<->git edit silently lost the UI side. The git->Docmost 3-way merge
   ran against a live Y.Doc that hadn't yet received the user's debounced in-flight
   edit, so git clean-applied (no conflict detected) and the edit vanished even on a
   different block. Fix: flush the pending debounced store before the merge so the
   in-flight edit is drained into the live doc first — a different-block edit is
   merged, a same-block one is detected and pinned to history (recoverable).

3. Smart-HTTP HEAD flapped to the read-only `docmost` mirror (~1/4 of clones). The
   engine transiently checks out `docmost` mid-pull and the host advertises whatever
   HEAD resolves to. Fix: VaultGit.pinHeadToMain(); the cycle restores HEAD->main in
   a finally; and the upload-pack ref advertisement is served HEAD-pinned under the
   per-space lock so it can never observe a mid-cycle HEAD.

4. (callout) clampCalloutType now mirrors the editor's GITHUB_ALERT_TYPE_MAP for
   non-schema aliases (tip->success, caution->danger, important->info) instead of
   flatly collapsing to info. The editor schema genuinely supports only the six
   banner types, so unknown types still fall back to info (by design).

Tests: deterministic real-git trailing-blank round-trip (no conflict, no markers,
in sync over 2 cycles) + genuine-conflict no-marker-leak; HEAD advertisement
stability; pre/post-flush concurrent-edit survival; serveReadAdvertisement lock
pin; widened callout-alias coverage. Engine vitest + server tsc + collaboration /
git-http / orchestrator specs all green.

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

apps/server/src/integrations/git-sync/http/git-http.service.ts

@@ -376,7 +376,25 @@ export class GitHttpService implements OnModuleDestroy {
     const isReceivePack =
       req.method === 'POST' && parsedPath.subpath === 'git-receive-pack';
     if (serviceKind === 'read' || !isReceivePack) {
-      await this.backend.run(backendRequest, rawReq, rawRes);
+      // The clone's default branch comes from the HEAD symref advertised by the
+      // upload-pack ref advertisement (or a dumb `GET HEAD`). The engine
+      // transiently checks out the read-only `docmost` mirror mid-cycle, so serve
+      // THAT advertisement with HEAD pinned to `main` under the per-space lock so
+      // a clone never defaults to `docmost` (bug #3). Pack streaming and every
+      // other read are resolved by object SHA and need no pin, so they stream
+      // directly (no lock) as before.
+      const isReadAdvertise =
+        req.method === 'GET' &&
+        ((parsedPath.subpath === 'info/refs' &&
+          service === 'git-upload-pack') ||
+          parsedPath.subpath === 'HEAD');
+      if (isReadAdvertise) {
+        await this.orchestrator.serveReadAdvertisement(spaceId, () =>
+          this.backend.run(backendRequest, rawReq, rawRes),
+        );
+      } else {
+        await this.backend.run(backendRequest, rawReq, rawRes);
+      }
       return;
     }