Staleness polling and untracked files disabled for bare ref mode (--base <sha>)

[belskikh]

When running diffity --base (single ref, no range), two features are silently disabled:

1. Staleness auto-refresh — getRefCapabilities() in packages/git/src/repo.ts only enables staleness for working-tree refs (work, ., staged, unstaged) and range refs (containing ..). A bare SHA/branch ref returns staleness: false, so the 3-second fingerprint poll never starts. The UI won't detect file changes until a manual browser refresh.
2. Untracked files — resolveDiffArgs() in packages/git/src/diff.ts sets includeUntracked: false for the default (bare ref) case. New files don't appear in the diff until git add.

This matters for worktree-based workflows where you run diffity --base to see the full branch diff (committed + uncommitted). Without these, the diff viewer appears broken — it doesn't refresh and misses new files.