docs(git-sync): add implementation spec for embedding docmost-sync
Detailed, signature-grounded implementation spec for the native in-process
git-sync feature: GitmostDataSource adapter mapping the engine's DocmostClient
subset onto PageRepo/SpaceRepo/PageService and the collab openDirectConnection
write path, per-space settings + UI, 'git-sync' provenance, Redis leader-lock,
event-driven + interval scheduling, repo-per-space vault topology, phasing A-D,
testing (round-trip idempotency gate vs editor-ext schema), risks and a
file-by-file change checklist. Specced against docmost-sync gitea main (b03eb35).
cecb560fce