fix(git-sync): push 503 starvation + concurrent-edit marker leak/silent loss

Bug #1 (push 503 starvation): an external receive-pack that briefly overlapped
a poll cycle immediately 503'd because the per-space single-writer lock was
held. Add a BOUNDED retry-acquire on the PUSH path only (SpaceLockService
.withSpaceLock acquireRetry: capped exponential backoff up to ~5s); a transient
overlap now waits and succeeds, a genuinely stuck cycle still 503s after the
bound. The poll cycle passes no retry (immediate skip). Push result stays
deterministic: the receive-pack only runs once the lock is held, so a 503 never
leaves a half-applied ref.

Bug #2 (concurrent-edit marker leak + silent same-block loss):
- Marker leak (a): the push UPDATE path stripped markers for the body sent to
  Docmost but left raw <<<<<<</>>>>>>> committed on the published `main` vault
  forever (autoMergeConflicts ON). Now the cleaned body is written back to the
  vault file + recorded in writtenBack so runPush commits it on `main` and the
  vault converges to clean bytes.
- Marker leak (b): pin merge.conflictStyle=merge in ensureRepo and teach
  stripConflictMarkers/hasConflictMarkers about the diff3 `|||||||` base section
  (drop the marker AND the stale base region) so diff3/zdiff3 conflicts can
  never leak `|||||||` + base content into a page. Also scrub the 3-way merge
  BASE markdown.
- Silent same-block loss: the block 3-way merge still resolves same-block
  conflicts deterministically to git, but it is no longer silent: diff3Plan now
  reports a conflict count (mergeXmlFragments3WayWithStats), gitSyncWriteBody
  logs it, and the persistence boundary-snapshot now fires for git-sync writes
  over a non-git-sync baseline so the human's pre-merge content is preserved in
  page history (recoverable). Full both-preserved persisted-conflict UI remains
  the deferred redesign.

Tests: space-lock bounded-retry (success/stuck/poll-immediate); push vault-clean
+ diff3 |||||||  strip; ensureRepo conflictStyle pin; diff3Plan/3-way conflict
counts; persistence git-sync boundary snapshot. Server tsc clean; git-sync
vitest + server collaboration/git-sync jest all green.

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

packages/git-sync/src/engine/git.ts

@@ -220,6 +220,13 @@ export class VaultGit {
     //     that core.autocrlf=false does not cover). POSIX-only path, which is
     //     fine: the daemon runs on Linux (Docker) / macOS. A system
     //     /etc/gitattributes remains the host admin's domain (out of scope).
+    //   - merge.conflictStyle=merge — CRITICAL (SPEC §9, conflict-marker leak):
+    //     a global `merge.conflictStyle=diff3`/`zdiff3` makes a conflicting merge
+    //     emit an EXTRA `|||||||` base-marker section. The conflict-marker
+    //     scrub on the push side (`stripConflictMarkers`) handles `|||||||` too,
+    //     but pinning the classic `merge` style keeps the markers the engine
+    //     produces to the canonical three (`<<<<<<<`/`=======`/`>>>>>>>`) so
+    //     behavior is deterministic regardless of the operator's global config.
     // NOTE: these stay PERSISTED LOCAL config (not `-c` flags) on purpose — a
     // human running git by hand in the vault must inherit the same neutralized
     // behavior; a transient `-c` would not persist. (core.quotepath, by
@@ -230,6 +237,7 @@ export class VaultGit {
       await this.run(["config", "core.safecrlf", "false"]);
       await this.run(["config", "commit.gpgsign", "false"]);
       await this.run(["config", "core.attributesFile", "/dev/null"]);
+      await this.run(["config", "merge.conflictStyle", "merge"]);
     } catch (err: unknown) {
       const detail = err instanceof Error ? err.message : String(err);
       throw new Error(