GitSync: honor arbitrary clone depth on sync, not just --depth 1

[tony]

Summary

GitSync can express shallow vs. full but not an arbitrary clone depth. GitSync.obtain() hardcodes the depth to 1 whenever git_shallow is set, even though the lower-level Git.clone() already accepts any depth. Exposing a depth attribute on GitSync would let callers request --depth N.

Current behavior (v0.41.0)

• GitSync.obtain() passes a fixed depth: src/libvcs/sync/git.py L370 — depth=1 if self.git_shallow else None.
• git_shallow is the only shallow control read from kwargs: src/libvcs/sync/git.py L261-L262.
• The clone command already supports arbitrary depth: src/libvcs/cmd/git.py L368 — depth: int | None = None.

Proposal

1. Read an optional depth: int from GitSync kwargs (alongside git_shallow).
2. In obtain(), pass depth=self.depth to Git.clone() when set, falling back to the current 1 if git_shallow else None behavior for compatibility.
3. Optionally let update_repo()/fetch() deepen or unshallow when depth changes, mirroring Git.fetch(depth=..., unshallow=...).

Why

Downstream tools (e.g. vcspull) can record a per-repository shallow state today, but only as a boolean because the sync path can't honor a numeric depth. A depth attribute on GitSync would let them persist and apply --depth N.