Hunk staging

Use this only when whole-path staging would mix independently reviewable changes.

1. Inspect a nonempty index with git diff --cached; preserve user-staged content unless the user requests otherwise.
2. For an untracked file, run git add -N -- <path> so its hunks become selectable.
3. Run git hunks list, then stage quoted IDs with git hunks add '<id>'....
4. Stage any complete files with git add -- <paths>.
5. Verify the complete staged result with git diff --cached.
6. Run commit save without paths. Paths would reset the index and discard the partial selection.

Hunk IDs remain stable while other hunks are staged, but not after editing the worktree. A failed multi-hunk add may stage earlier requested hunks, so always perform the final verification.