fix: cherry-pick --continue fails when AI resolution produces empty commit

[myakove]

Problem

After AI resolves cherry-pick conflicts, cherry-pick --continue can fail with:
"The previous cherry-pick is now empty, possibly due to conflict resolution."

This happens when the AI resolves the conflict by choosing the target branch's version entirely, resulting in no actual code changes (empty commit).

Expected Behavior

The server should handle empty cherry-pick results by using git commit --allow-empty -C CHERRY_PICK_HEAD.

Observed Behavior

Cherry-pick fails and posts manual instructions even though AI resolved the conflicts successfully.

Deliverables

• Handle "cherry-pick is now empty" error with --allow-empty commit
• Add test for empty cherry-pick scenario