feat(advisor): add 0.85 advisory threshold triggering LLM suggestions

- Add advisory_threshold=0.85 field to MetricRule (higher-is-better metrics)
- diagnose() now emits severity='low' for scores in (warning_threshold, 0.85)
- noise_sensitivity (lower-is-better) keeps its existing two-tier thresholds
- writer.py: severity labels mapped to Chinese (严重/警告/待优化)
- llm_analyzer.py: prompt explains low/warning/critical tiers in Chinese
- Tests: 5 new cases for 'low' severity, updated log summary assertions

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

tests/test_advisor_writer.py

@@ -91,9 +91,9 @@ class TestWriteAdvice(unittest.TestCase):
         ]
         summary = _format_log_summary(diags, self.advice_path)
         self.assertIn("faithfulness", summary)
-        self.assertIn("critical", summary)
+        self.assertIn("严重", summary)   # "critical" maps to Chinese label
         self.assertIn("context_recall", summary)
-        self.assertIn("warning", summary)
+        self.assertIn("警告", summary)   # "warning" maps to Chinese label
 
     def test_write_empty_diagnoses_still_creates_file(self):
         write_advice(