fix: capture_diff uses base commit to handle agent self-commits

Claude in agentic mode (interactive, no -p flag) commits its own changes,
advancing HEAD. This made `git diff --cached HEAD` return empty, triggering
false EMPTY_DIFF errors every time. Now capture_diff diffs against the
base commit SHA recorded at worktree creation, so changes are captured
regardless of whether the agent committed them.

Also adds UX_IMPROVEMENT_PLAN.md for guided message improvements.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

cross_eval/agent.py

@@ -414,6 +414,7 @@ def invoke_agent_agentic(
     env: Optional[dict[str, str]] = None,
     timeout: int | None = None,
     quiet: bool = False,
+    base_commit: str | None = None,
 ) -> AgentResult:
     """Invoke an agent in agentic mode using the worktree as the source of truth."""
     from cross_eval.worktree import capture_diff
@@ -506,8 +507,8 @@ def invoke_agent_agentic(
             suggested_action=suggested_action,
         )
 
-    # Capture git diff as the output (changes since last commit on the branch)
-    diff_output = capture_diff(worktree_path)
+    # Capture git diff as the output (changes since the base commit)
+    diff_output = capture_diff(worktree_path, base_commit=base_commit)
 
     if not diff_output:
         stdout_excerpt = (result.stdout or "").strip()