fix(git-sync): address PR #119 review #4 — symlink guard, dead-code cull, changelog + warnings/suggestions

Blocking (review id 2514):
- [security] Forbid symlinks in vaults. ensureServable now sets
  core.symlinks=false in each vault's local git config (a pushed symlink is
  checked out as a plain file, never a real link), and the engine cycle wraps
  every read/write/mkdir in an lstat/realpath guard (new path-guard.ts) that
  refuses a path that is — or traverses — a symlink, or whose realpath escapes
  the vault root. Prevents a writer from publishing /etc/passwd or the server
  .env, or writing outside the vault. Adds unit tests (path-guard.test.ts) +
  a read-guard integration test (cycle.test.ts) + real lstat/realpath in the
  roundtrip integration test.
- [simplification] Delete dead lib/diff.ts + test/diff.test.ts and drop the
  now-unused @fellow/prosemirror-recreate-transform dependency.
- [documentation] Add a CHANGELOG [Unreleased] → Added entry for git-sync.

Warnings:
- [test-coverage] Cover the CREATE-branch conflict-markers guard (a new .md with
  markers and no gitmost_id is recorded as a create failure, never created).

Suggestions:
- [stability] Bound each `git config` in ensureServable with a timeout.
- [authz] Trigger endpoint resolves spaceId workspace-scoped and 404s a foreign
  space before any vault directory is created.
- [stability] Attribute git-initiated moves to the service account
  (lastUpdatedById), via an optional actor param on PageService.movePage.
- [documentation] Document the per-space autoMergeConflicts toggle in AGENTS.md.
- [test-coverage] Cover the unterminated `:::` callout fence fallback.
- [simplification] Move test-only roundtrip-helpers.ts out of src/ into test/.

Architecture:
- Move the Yjs/ProseMirror merge primitives (yjs-body-merge, three-way-merge,
  lcs + specs) into collaboration/merge/, breaking the collaboration →
  integrations/git-sync dependency cycle this PR introduced.
- Port the schema-surface drift gate to packages/mcp (the mcp schema mirror had
  none); pins 52 entries.

Deferred (with rationale in the review thread): the incremental-pull perf
warning (correctness-neutral; needs a high-water-mark design + its own tests on
the data-loss-critical path) and the redis-sync rolling-deploy mixed-version
edge (the deficient behavior is in already-released old-instance code; the new
code is correct on both sides; impact is a transient rollout-window artifact).

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

packages/git-sync/test/redteam-push-cycle.test.ts

@@ -1,5 +1,10 @@
 import { describe, expect, it, vi } from 'vitest';
-import { runPush, LAST_PUSHED_REF, DOCMOST_BRANCH } from '../src/engine/push';
+import {
+  runPush,
+  LAST_PUSHED_REF,
+  DOCMOST_BRANCH,
+  CONFLICT_MARKERS_FAILURE_REASON,
+} from '../src/engine/push';
 import type { PushDeps } from '../src/engine/push';
 import type { Settings } from '../src/engine/settings';
 import { runCycle, type RunCycleDeps } from '../src/engine/cycle';
@@ -139,6 +144,59 @@ describe('#13 conflict markers reach Docmost', () => {
     expect(pushedBody).toContain('my line');
     expect(pushedBody).toContain('their line');
   });
+
+  it('CREATE branch (autoMergeConflicts off): does NOT create a page from a conflicted NEW file; records a create failure', async () => {
+    // The conflict-markers guard is DUPLICATED on the CREATE path (a brand-new
+    // .md with NO gitmost_id, status 'A') and was previously untested — only the
+    // UPDATE branch had coverage. Without this, a regression would SILENTLY push
+    // `<<<<<<<`/`>>>>>>>` into a freshly-created page. Assert the create path
+    // isolates it exactly like update: no createPage, a kind:'create' failure
+    // with the conflict reason, and the refs held.
+    const { git, calls } = makePushGit({
+      changes: [{ status: 'A', path: 'New.md' }],
+    });
+    const createPage = vi.fn(async () => ({ data: { id: 'new-1' } }));
+    const client = {
+      listSpaceTree: vi.fn(async () => ({ pages: [], complete: true })),
+      importPageMarkdown: vi.fn(),
+      createPage,
+      deletePage: vi.fn(),
+      movePage: vi.fn(),
+      renamePage: vi.fn(),
+    };
+    const deps: PushDeps = {
+      // makeSettings() leaves autoMergeConflicts undefined -> the SAFE default.
+      settings: makeSettings(),
+      git,
+      makeClient: () => client as any,
+      // Raw conflict body with NO gitmost_id frontmatter -> classified as CREATE.
+      readFile: vi.fn(async (path: string) => {
+        if (path === 'New.md') return conflictBody;
+        throw new Error(`no such file: ${path}`);
+      }),
+      writeFile: vi.fn(async () => {}),
+      log: () => {},
+    };
+
+    const res = await runPush(deps, { dryRun: false });
+    expect(res.mode).toBe('apply');
+
+    // No page was created from the conflicted content.
+    expect(createPage).not.toHaveBeenCalled();
+
+    // Recorded as a CREATE failure with the conflict-markers reason.
+    expect(res.applied?.failures).toEqual([
+      expect.objectContaining({
+        kind: 'create',
+        path: 'New.md',
+        error: CONFLICT_MARKERS_FAILURE_REASON,
+      }),
+    ]);
+
+    // A failure prevents advancing the last-pushed ref.
+    expect(res.applied?.lastPushedAdvanced).toBe(false);
+    expect(calls.updateRef).toHaveLength(0);
+  });
 });
 
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