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/worktree.py

@@ -37,18 +37,31 @@ def make_worktree_dir(base_cwd: Path, branch_name: str) -> Path:
     )
 
 
-def create_worktree(base_cwd: Path, work_dir: Path, branch_name: str) -> Path:
+def create_worktree(base_cwd: Path, work_dir: Path, branch_name: str) -> tuple[Path, str]:
     """Create a git worktree on a new branch from HEAD.
 
     1. Create branch from HEAD
     2. Create worktree checked out to that branch
 
     The branch lives in the original repo, so it survives worktree removal.
+    Returns (worktree_path, base_commit_sha).
     """
     work_dir = work_dir.resolve()
     if work_dir.exists():
         shutil.rmtree(work_dir)
 
+    # Record the base commit SHA before creating the branch.
+    # This is the anchor for all diffs — even if the agent makes its own commits,
+    # we always diff against this base to capture the full set of changes.
+    result = subprocess.run(
+        ["git", "rev-parse", "HEAD"],
+        cwd=base_cwd,
+        capture_output=True,
+        text=True,
+        check=True,
+    )
+    base_commit = result.stdout.strip()
+
     # Create the branch at HEAD
     try:
         subprocess.run(
@@ -83,15 +96,24 @@ def create_worktree(base_cwd: Path, work_dir: Path, branch_name: str) -> Path:
             f"Failed to create worktree at {work_dir}: {e.stderr.strip()}"
         ) from e
 
-    logger.debug("Created worktree on branch '%s': %s", branch_name, work_dir)
-    return work_dir
+    logger.debug("Created worktree on branch '%s': %s (base: %s)", branch_name, work_dir, base_commit[:8])
+    return work_dir, base_commit
 
 
-def capture_diff(worktree_path: Path) -> str:
+def capture_diff(worktree_path: Path, base_commit: str | None = None) -> str:
     """Capture all changes made in the worktree as a unified diff.
 
-    Includes both tracked modifications and new untracked files.
+    Includes both tracked modifications, new untracked files, and changes
+    that the agent may have committed on its own.
+
+    Args:
+        base_commit: The commit SHA from when the worktree was created.
+                     If provided, diffs against this fixed base instead of HEAD.
+                     This is critical because agents (e.g. Claude in interactive
+                     mode) may create their own commits, advancing HEAD and
+                     making ``git diff --cached HEAD`` return empty.
     """
+    # Stage any uncommitted changes so they're included in the diff
     subprocess.run(
         ["git", "add", "-A"],
         cwd=worktree_path,
@@ -99,6 +121,30 @@ def capture_diff(worktree_path: Path) -> str:
         check=True,
     )
 
+    if base_commit:
+        # Diff everything (committed + staged) against the original base.
+        # This captures changes regardless of whether the agent committed them.
+        result = subprocess.run(
+            ["git", "diff", base_commit, "--cached"],
+            cwd=worktree_path,
+            capture_output=True,
+            text=True,
+        )
+        diff = result.stdout.strip()
+        if diff:
+            return diff
+
+        # Also check committed changes (agent may have committed and left
+        # nothing staged)
+        result = subprocess.run(
+            ["git", "diff", base_commit, "HEAD"],
+            cwd=worktree_path,
+            capture_output=True,
+            text=True,
+        )
+        return result.stdout.strip()
+
+    # Fallback: no base_commit, use original behavior
     result = subprocess.run(
         ["git", "diff", "--cached", "HEAD"],
         cwd=worktree_path,