fix(opencode_review): harden _synthesize_summary_fields (none-safe, dedupe emoji map)

tests/pilot/test_opencode_review.py

@@ -905,3 +905,47 @@ def test_synthesize_test_coverage_missing_tests():
     _, _, tc = oc._synthesize_summary_fields(
         [], "+diff\n", changed_paths=["pilot/foo.py"])
     assert "No tests for behavioral change" in tc
+
+
+def test_synthesize_walkthrough_picks_peak_severity_per_path():
+    # Three findings on the same path, with mixed severities. The walkthrough
+    # headline should use the PEAK severity's emoji (critical = 🔴), not the
+    # lexicographic-first severity (low).
+    findings = [
+        {"path": "x.py", "line": 1, "severity": "low",
+         "problem": "minor nit"},
+        {"path": "x.py", "line": 5, "severity": "critical",
+         "problem": "sql injection"},
+        {"path": "x.py", "line": 9, "severity": "high",
+         "problem": "auth bypass"},
+    ]
+    w, _, _ = oc._synthesize_summary_fields(findings, "")
+    assert len(w) == 1
+    line = w[0]
+    assert "`x.py`" in line
+    assert "🔴" in line        # critical = 🔴
+    assert "🟡" not in line
+    assert "🔵" not in line
+    assert "sql injection" in line  # critical finding's problem, not low's
+
+
+def test_synthesize_summary_fields_none_findings_safe():
+    # Old code crashed in risk_verdict with `for f in findings:` on None.
+    # After the `findings = findings or []` guard, None behaves like [].
+    w, rv, tc = oc._synthesize_summary_fields(None, "")
+    assert isinstance(w, list)
+    assert rv.startswith("Low risk")
+    # walkthrough should fall through to the diff-derived path list — empty
+    # diff produces no lines, but no crash is the point.
+    assert tc == ""
+
+
+def test_synthesize_walkthrough_empty_problem_does_not_crash():
+    # An empty `problem` should render as "`a.py` — emoji" with a trailing
+    # space, not raise. Regression guard for splitlines()[0][:80].strip().
+    findings = [{"path": "a.py", "line": 1,
+                 "severity": "low", "problem": ""}]
+    w, _, _ = oc._synthesize_summary_fields(findings, "")
+    assert len(w) == 1
+    assert "`a.py`" in w[0]
+    assert "🔵" in w[0]  # low severity emoji