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>

apps/server/src/integrations/git-sync/git-sync.controller.ts

@@ -4,6 +4,7 @@ import {
   ForbiddenException,
   HttpCode,
   HttpStatus,
+  NotFoundException,
   Post,
   Get,
   UseGuards,
@@ -12,6 +13,7 @@ import { JwtAuthGuard } from '../../common/guards/jwt-auth.guard';
 import { AuthUser } from '../../common/decorators/auth-user.decorator';
 import { AuthWorkspace } from '../../common/decorators/auth-workspace.decorator';
 import { User, Workspace } from '@docmost/db/types/entity.types';
+import { SpaceRepo } from '@docmost/db/repos/space/space.repo';
 import WorkspaceAbilityFactory from '../../core/casl/abilities/workspace-ability.factory';
 import {
   WorkspaceCaslAction,
@@ -47,6 +49,7 @@ export class GitSyncController {
     private readonly orchestrator: GitSyncOrchestrator,
     private readonly environmentService: EnvironmentService,
     private readonly workspaceAbility: WorkspaceAbilityFactory,
+    private readonly spaceRepo: SpaceRepo,
   ) {}
 
   /** Throw unless the caller is a workspace admin (Manage Settings). */
@@ -67,6 +70,15 @@ export class GitSyncController {
     @AuthWorkspace() workspace: Workspace,
   ): Promise<GitSyncRunStatus> {
     this.assertAdmin(user, workspace);
+    // Verify the client-supplied spaceId BELONGS to this workspace before doing
+    // any work (review): without this, `runOnce` -> `buildSettings` reads the
+    // raw `spaces` row and creates an empty per-space vault directory for a
+    // foreign/non-existent space before the content read finally 404s. Resolve
+    // it workspace-scoped and 404 early.
+    const space = await this.spaceRepo.findById(dto.spaceId, workspace.id);
+    if (!space) {
+      throw new NotFoundException('Space not found');
+    }
     // Use the workspace from the request context (never client-supplied).
     return this.orchestrator.runOnce(dto.spaceId, workspace.id);
   }