fix(git-sync): review #4404 batch — sanitize-title echo, per-space gate, move-echo, merge-agreement, fence-aware conflict scan, e2e asserts

Addresses reviewer comment #4404 (critical + blocking):

- Critical #2: renamePage skips the echo where the incoming title equals
  sanitizeTitle(current title) — a Docmost title with FS-hostile chars (: / " |,
  newlines, double-space, >120) was pulled to a sanitized stem then written back,
  permanently corrupting the real title. (datasource)
- Blocking #3: runOnce enforces per-space settings.gitSync.enabled (the event
  path bypassed opt-in; any edited space would git-init + export). (orchestrator)
- Blocking #6: movePage no-ops the position-less same-parent echo that clobbered
  the user's chosen sibling order. (datasource)
- Blocking #9: hasConflictMarkers is fence-aware — '<<<<<<< HEAD' inside a code
  block (git-tutorial page) no longer trips the all-or-nothing gate that froze
  the whole space's refs. (push.ts)
- Blocking #11: three-way tryMergeRegion short-circuits when live==target (diff3
  agreement) instead of logging a false 'same-block conflict resolved to git' —
  the echo noise that masked real data-loss signals. (three-way-merge)
- Blocking #12/#13: e2e-advanced — drop the delete-cap block (no such feature;
  failed with a scary '(data loss!)'); non-member assert now expects 404 (existence
  not leaked), not 403.

Verified on stand: sanitized-title rename preserves DB title (vault file
sanitized); non-enabled space creates no vault; fenced conflict markers ingest
without jamming; build clean.

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

apps/server/src/collaboration/merge/three-way-merge.ts

@@ -125,6 +125,16 @@ function tryMergeRegion(
   a: string[],
   b: string[],
 ): LocalPick[] | null {
+  // Agreement short-circuit (review #11). When live (a) and target (b) are
+  // identical, both sides converged on the SAME result — diff3 "agreement", NOT
+  // a conflict. This is the dominant echo case (live == target != base) that
+  // otherwise trips the overlap check below and is logged as a false "N same-block
+  // conflict(s) resolved to the git version", masking REAL data-loss signals.
+  // Emit the region straight from live (which equals target); no conflict.
+  if (a.length === b.length && a.every((v, i) => v === b[i])) {
+    return a.map((_v, i) => ({ src: 'live', local: i }) as LocalPick);
+  }
+
   const aHunks = buildHunks(o, a);
   const bHunks = buildHunks(o, b);