feat: discard working-tree changes

[uditdc]

Scope
Add "Discard" action per-file and for all changes in the working-tree panel context menu (git restore for tracked, git clean for untracked), each behind a confirmation dialog.

Acceptance criteria

• Discarding a modified tracked file reverts it to HEAD's version.
• Discarding untracked files requires a separate, explicit confirmation (different/higher blast radius than restoring tracked files).