fix: use incremental diff per iteration instead of cumulative base diff

After each iteration's _commit_iteration, record the new HEAD SHA and use
it as the diff anchor for the next iteration. Previously capture_diff
always diffed against the initial base commit, causing every iteration to
return the same full cumulative diff — reviewers couldn't see what changed
between iterations, leading to repeated feedback and stuck FAIL loops.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

cross_eval/pipeline.py

@@ -62,18 +62,20 @@ def _commit_iteration(
     label: str,
     iteration: int,
     verdict: str | None,
-) -> None:
+) -> str:
     """Intermediate commit after each agentic iteration.
 
     This resets the diff baseline so the next iteration only captures new changes.
+    Returns the new HEAD SHA to use as the base_commit for the next iteration.
     """
-    from cross_eval.worktree import commit_worktree
+    from cross_eval.worktree import commit_worktree, get_current_head
     committed = commit_worktree(
         worktree_path,
         f"cross-eval: {label} v{iteration} ({verdict or 'no-verdict'})",
     )
     if committed:
         logger.debug("  Intermediate commit: v%d (%s)", iteration, verdict)
+    return get_current_head(worktree_path)
 
 
 def _has_agentic_steps(config: PipelineConfig, steps: list[StepConfig]) -> bool:
@@ -388,7 +390,7 @@ def _run_simple_pipeline(
 
             # Intermediate commit so next iteration's diff only shows new changes
             if worktree_path is not None:
-                _commit_iteration(worktree_path, config.preset_name, i, verdict)
+                agentic_base_commit = _commit_iteration(worktree_path, config.preset_name, i, verdict)
 
             iter_result = IterationResult(
                 iteration=i,
@@ -588,7 +590,7 @@ def _run_phased_pipeline(
 
                 # Intermediate commit so next iteration's diff only shows new changes
                 if worktree_path is not None:
-                    _commit_iteration(
+                    agentic_base_commit = _commit_iteration(
                         worktree_path, f"{config.preset_name}/{phase.name}",
                         global_iter, verdict,
                     )