Add Virtual Git Context System (VGCS) crate to handle versioned file storage backed by git2 and blob store.
Features:
- ContextStore trait for repo init, read, list, diff, merge.
- Transaction trait for atomic writes and commits.
- Large file support (>1MB) using content-addressed blob store.
- In-memory merge with conflict detection.
- Unit and integration tests covering concurrency and conflicts.
