fix(git-sync): red-team hardening — 12 confirmed sync-breaking bugs + regression tests

A 10-agent red-team pass on the two-way Docmost<->git sync surfaced 16 ranked
findings (9 others triaged out as already-defended). Wrote a reproduction test
per finding (each asserts the CORRECT behavior, so it fails on the bug), then
fixed the production code so every repro goes green. All confirmed bugs:

Round-trip data loss (markdown-converter.ts + docmost-schema.ts mirror):
- #1 editor-ext node types silently dropped on export — ported the 8 missing
  canon nodes (footnoteReference/footnotesList/footnoteDefinition, htmlEmbed,
  status, pageEmbed, transclusionSource/Reference) into the git-sync schema
  mirror and added converter cases that emit their schema-matching HTML instead
  of flattening unknown nodes to '' (this was the critical data-loss flagged in
  review #1679: footnotes/htmlEmbed lost on sync). Snapshot surface updated.
- #2 top-level image lost width/height/align/attachmentId — now emits an HTML
  <img> (like video/diagrams) when it carries layout attrs; bare images stay
  ![](src). Image node parses width/height as strings so they re-import.
- #3 code block containing a ``` fence corrupted on round-trip — outer fence is
  now widened to (longest-inner-backtick-run + 1).
- #16 deep nesting threw RangeError (page never synced) — added a depth guard
  (MAX_NODE_DEPTH=400) so the converter never overflows the stack.

Push/layout/cycle (engine):
- #4 disambiguation ' ~slugId' suffix corrupted Docmost titles + order-dependent
  layout — deterministic, order-independent sibling disambiguation; suffix is
  stripped from a path-derived title ONLY when the new name is exactly the old
  title plus the suffix (never a genuine retitle ending in ' ~token').
- #6 retry-adopt by (parent,title) clobbered the wrong duplicate-title sibling —
  ambiguous (parent,title) is no longer adopted (falls back to fresh create).
- #12 a new child under a new parent was created at ROOT — creates are ordered
  parent-before-child with an in-memory created-id map for parent resolution.
- #13 git conflict markers could reach Docmost — bodies are scanned and the
  marker lines stripped (a '=======' line is only treated as a conflict
  separator inside a <<<<<<< ... >>>>>>> block, so setext headings are safe).
- #15 a divergent `docmost` mirror was escalated by runPush but dropped by
  runCycle — RunCycleResult now forwards divergentDocmost to the orchestrator.

Server (merge / lock / provenance):
- #9 3-way merge lost a human's block edit when git inserted an adjacent block —
  finer-grained diff3 region merge (via lcs) preserves non-overlapping human
  edits; genuine same-block conflicts still resolve git-wins.
- #10 single-writer race — module-static liveLocks closes the same-process TOCTOU
  window, and a heartbeat refresh that cannot confirm the lock now aborts the
  cycle at its next write checkpoint (cooperative AbortSignal threaded through
  runCycle). Cross-process fencing tokens remain a follow-up.
- #14 sticky-agent provenance overrode an explicit actor='git-sync' write,
  blinding the listener loop-guard — resolveSource now lets an explicit actor
  win over the sticky-agent fallback (explicit agent still wins).

Verified: git-sync vitest 617 pass (+1 expected-fail), server unit jest 1541
pass, server tsc clean. A review pass over the fixes caught and corrected a
title-suffix over-strip, an inert abort signal, a document-wide conflict-marker
strip, and two leaf-atom content-holes.

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

apps/server/src/integrations/git-sync/services/git-sync.orchestrator.ts

@@ -151,8 +151,8 @@ export class GitSyncOrchestrator implements OnModuleInit, OnModuleDestroy {
     // when it could not enter — surfaced here as the existing skipped:'in-progress'
     // / 'lock-held' status so runOnce's observable behavior is unchanged.
     try {
-      const result = await this.spaceLock.withSpaceLock(spaceId, () =>
-        this.driveCycle(spaceId, workspaceId, serviceUserId),
+      const result = await this.spaceLock.withSpaceLock(spaceId, (signal) =>
+        this.driveCycle(spaceId, workspaceId, serviceUserId, signal),
       );
       if ('skipped' in result && !('spaceId' in result)) {
         return { spaceId, ran: false, skipped: result.skipped };
@@ -199,7 +199,7 @@ export class GitSyncOrchestrator implements OnModuleInit, OnModuleDestroy {
     }
     const serviceUserId = this.environmentService.getGitSyncServiceUserId();
 
-    const result = await this.spaceLock.withSpaceLock(spaceId, async () => {
+    const result = await this.spaceLock.withSpaceLock(spaceId, async (signal) => {
       // 1) Stream the receive-pack to the client (durable commits land on main).
       await runReceivePack();
 
@@ -214,7 +214,7 @@ export class GitSyncOrchestrator implements OnModuleInit, OnModuleDestroy {
         return;
       }
       try {
-        await this.driveCycle(spaceId, workspaceId, serviceUserId);
+        await this.driveCycle(spaceId, workspaceId, serviceUserId, signal);
       } catch (err) {
         // Do NOT rethrow: the push succeeded and the commits are durable on main;
         // the poll-interval backstop retries the cycle. Log for visibility.
@@ -246,6 +246,7 @@ export class GitSyncOrchestrator implements OnModuleInit, OnModuleDestroy {
     spaceId: string,
     workspaceId: string,
     serviceUserId: string,
+    signal?: AbortSignal,
   ): Promise<GitSyncRunStatus> {
     const { runCycle } = await loadGitSync();
     const settings = this.buildSettings(spaceId);
@@ -254,6 +255,10 @@ export class GitSyncOrchestrator implements OnModuleInit, OnModuleDestroy {
     const maxDeletes = this.environmentService.getGitSyncMaxDeletesPerCycle();
 
     const result = await runCycle({
+      // Cooperative-abort signal from the per-space lock: if a heartbeat refresh
+      // cannot confirm the lock, the cycle bails before its next destructive
+      // write phase instead of writing blind after a possible lock loss.
+      signal,
       spaceId,
       client,
       vault,