chore(agents): improve ca-subtask-loop — add meaningful-change verification

Approved proposal: #2443
Pattern: workflow_fix
Evidence: PR #1513 contained only a single comment line as its entire
diff, yet passed all quality gates (lint, typecheck, tests, coverage)
and was submitted for review, consuming 8 independent reviewer slots.
All quality gates verify code quality, not code existence — a change
that adds only comments passes every gate.
Fix: Add Step 1.5 (Meaningful Change Verification) between the
implementer invocation and test-writing step. This checks git diff
output for functional code changes and rejects attempts that produce
empty, comment-only, or trivially small diffs (< 3 functional lines),
immediately escalating to the next tier without wasting quality gate
and reviewer capacity.

ISSUES CLOSED: #2443

.opencode/agents/ca-subtask-loop.md

@@ -109,6 +109,38 @@ Loop FOREVER:
     Pass: working directory, ref summary, subtask description,
           spec context, issue details, attempt_log (if attempt > 1)
 
+  # Step 1.5: Meaningful Change Verification
+  # Before running expensive quality gates, verify the implementer
+  # actually produced functional code changes — not just comments,
+  # whitespace, or empty diffs.
+  diff_output = run `git diff --stat` in working directory
+  full_diff = run `git diff` in working directory
+
+  reject_attempt = false
+  if diff_output is empty:
+    reject_attempt = true  # No changes at all
+  else:
+    # Analyze the diff content for meaningful changes
+    functional_lines = count lines in full_diff that:
+      - Are additions (start with "+") or deletions (start with "-")
+      - Are NOT comment-only lines (e.g., lines where the only content
+        after +/- is a Python comment starting with #)
+      - Are NOT whitespace-only changes
+      - Are NOT empty lines
+      - Are NOT import-only additions with no corresponding usage
+    if functional_lines < 3:
+      reject_attempt = true  # Trivially small or comment-only change
+
+  if reject_attempt:
+    Record in attempt_log:
+      "Attempt <N> rejected: implementer produced no meaningful code
+       changes (empty diff, comment-only, or fewer than 3 functional
+       lines). Skipping quality gates and escalating."
+    # Reset the working directory to avoid polluting the next attempt
+    run `git checkout -- .` in working directory
+    # Continue to next attempt (escalation) WITHOUT running quality gates
+    continue
+
   # Step 2: Write / Review ALL Tests IN PARALLEL
   # Launch ALL test writers simultaneously — do not serialize.
   invoke ALL of the following IN PARALLEL: