v0.5.0 — git-likeness Tier 1

Rounds out the local VCS so it feels git-native, reusing the existing diff/patch/merge engines.

• Revision syntax — HEAD, branch/tag names, abbreviated hashes, and ~n / ^n ancestor suffixes everywhere (diff HEAD~2 HEAD, checkout main~1, show <short>).
• tag — create/list/-d, resolvable as revisions.
• show <ref> — a commit's metadata + its diff vs the first parent.
• reset <ref> [--hard] — move the current branch (and update the worktree with --hard).
• restore <ref> <path>... — materialize selected files/regions from a snapshot.
• revert <commit> — a new commit that undoes one, via the 3-way merge engine (conflicts reported).
• log — --oneline, -p (patch), --stat, -n.
• .mcaignore — gitignore-lite (*.ext, dir/, name, /anchored/path) honored on commit.

67 tests (was 61). Next: Tier 2 — filesystem remotes (clone/fetch/push), reflog, cherry-pick, gc.