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>

tests/test_runtime_misc.py

@@ -775,11 +775,18 @@ class TestRuntimeEnvironmentHelpers(unittest.TestCase):
 class TestWorktreeFailures(unittest.TestCase):
     @patch("cross_eval.worktree.subprocess.run")
     def test_create_worktree_raises_when_branch_creation_fails(self, mock_run: MagicMock) -> None:
-        mock_run.side_effect = subprocess.CalledProcessError(
-            1,
-            ["git", "branch"],
-            stderr="branch failed",
-        )
+        # First call: git rev-parse HEAD (succeeds)
+        # Second call: git branch (fails)
+        rev_parse_result = MagicMock(returncode=0)
+        rev_parse_result.stdout = "a" * 40
+        mock_run.side_effect = [
+            rev_parse_result,
+            subprocess.CalledProcessError(
+                1,
+                ["git", "branch"],
+                stderr="branch failed",
+            ),
+        ]
 
         with tempfile.TemporaryDirectory() as tmpdir:
             base = Path(tmpdir)
@@ -791,14 +798,17 @@ class TestWorktreeFailures(unittest.TestCase):
 
     @patch("cross_eval.worktree.subprocess.run")
     def test_create_worktree_cleans_branch_on_worktree_failure(self, mock_run: MagicMock) -> None:
+        rev_parse_result = MagicMock(returncode=0)
+        rev_parse_result.stdout = "a" * 40
         mock_run.side_effect = [
-            MagicMock(returncode=0),
+            rev_parse_result,           # git rev-parse HEAD
+            MagicMock(returncode=0),    # git branch
             subprocess.CalledProcessError(
                 1,
                 ["git", "worktree", "add"],
                 stderr="worktree failed",
             ),
-            MagicMock(returncode=0),
+            MagicMock(returncode=0),    # git branch -D (cleanup)
         ]
 
         with tempfile.TemporaryDirectory() as tmpdir: