vcs(feat[lock]): Implement version locking for repositories

why: Enable users to lock repositories to specific revisions for deployment consistency
     and reproducible environments. This allows teams to ensure all members are working
     with the same codebase state.

what:
- Added LockFile and LockedRepository models to config/models.py
- Implemented lock_repositories and apply_lock functions in operations.py
- Added get_revision and update_repo abstract methods to VCSInterface
- Implemented these methods for Git, Mercurial, and Subversion handlers
- Created CLI commands for locking repositories and applying locks
- Updated public exports in __init__.py files

refs: Completes CLI Tools / Version Locking section from notes/TODO.md

Author: tony

src/vcspull/__init__.py

@@ -16,16 +16,23 @@
 from . import cli
 from .__about__ import __author__, __description__, __version__
 from .config import load_config, resolve_includes
-from .operations import detect_repositories, sync_repositories
+from .operations import (
+    apply_lock,
+    detect_repositories,
+    lock_repositories,
+    sync_repositories,
+)
 
 logging.getLogger(__name__).addHandler(NullHandler())
 
 __all__ = [
     "__author__",
     "__description__",
     "__version__",
+    "apply_lock",
     "detect_repositories",
     "load_config",
+    "lock_repositories",
     "resolve_includes",
     "sync_repositories",
 ]