feat(git-sync): native GitmostDataSource + 'git-sync' provenance (Phase A.4a)

Native data plane for git-sync (plan §3, §8.1):
- provenance: widen actor to 'user'|'agent'|'git-sync' (jwt-payload,
  auth-provenance decorator); PersistenceExtension resolves lastUpdatedSource
  with precedence agent > git-sync > user, debounced history (like a human edit,
  not the agent's immediate snapshot).
- GitmostDataSourceService implements @docmost/git-sync's GitSyncClient natively:
  reads via PageRepo/SpaceRepo (listSpaceTree complete:true, getPageJson), writes
  via PageService (create/removePage soft-delete/movePage with computed fractional
  position/update-rename/restore) + the writeBody linchpin through collab
  openDirectConnection('page.'+id, {actor:'git-sync'}) mirroring
  collaboration.handler withYdocConnection 'replace'. bind({workspaceId,userId})
  returns the context-bound client for the orchestrator.
- 10 unit/contract tests (mapping + soft-delete + move-position), tsc clean.

Known gap (closed in A.4b): PageService.create/update/movePage only branch on
actor==='agent'; git-sync provenance is already passed through so the row source
marker propagates once PageService honors 'git-sync'. Module/orchestrator/config
come next.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

apps/server/package.json

@@ -40,6 +40,7 @@
     "@aws-sdk/s3-request-presigner": "3.1050.0",
     "@azure/storage-blob": "12.31.0",
     "@clickhouse/client": "^1.18.2",
+    "@docmost/git-sync": "workspace:*",
     "@docmost/mcp": "workspace:*",
     "@docmost/pdf-inspector": "1.9.6",
     "@fastify/cookie": "^11.0.2",