fix(severity): extend badge label set + update SYSTEM_PROMPT

tests/pilot/test_ai_review.py

@@ -17,6 +17,7 @@ from ai_review import (  # noqa: E402
     _last_balanced_json,
     _normalize_finding,
     _render_collapsible_usage,
+    _severity_badge,
     build_user_prompt,
     compute_attribution,
     findings_table,
@@ -321,7 +322,7 @@ def test_inline_comment_body_severity_emoji_mapping():
         ("medium", "🟡 [MEDIUM]"),
         ("low", "🔵 [LOW]"),
         ("info", "⚪ [INFO]"),
-        ("nit", "⚪ [INFO]"),  # "nit" maps to the INFO label
+        ("nit", "⚪ [NIT]"),  # legacy alias — renders with its own name
         ("bogus", "⚪ [INFO]"),  # unknown severity falls back to INFO
     ]
     for sev, badge in cases:
@@ -1746,3 +1747,11 @@ def test_emoji_for_trivial_and_info_is_neutral():
     assert _SEVERITY_EMOJI["trivial"] == "⚪"
     assert _SEVERITY_EMOJI["info"] == "⚪"
 
+
+def test_severity_badge_labels_each_known_severity():
+    # Trivial and info (and legacy nit) should render with their own name,
+    # not fall back to "INFO".
+    for sev in ("critical", "high", "medium", "low", "trivial", "info", "nit"):
+        badge = _severity_badge(sev)
+        assert f"[{sev.upper()}]" in badge, (sev, badge)
+