fix(orchestrator): P5 audit-anchor-first-line regression guard

Bug discovered during #56 INTEGRATION-AUDIT-02 execution (2026-05-20):
- Both Claude and Codex put "Audit anchor: ..." as the FIRST line of every
  Gitea comment per the #56 issue body instruction "cite anchor at start
  of every stage".
- detect_agent (P0-1 strict, first-line only) then returns None for these
  comments because the first line is "Audit anchor:..." not "[Codex #N]"
  or "[Claude #N]".
- Result: orchestrator's "is_codex" check (line ~1288) flips false →
  "Codex 응답 미감지 — continuing" → infinite Stage 4 loop. #56 reached
  Round #14 (>300 comments, ~2 hours wasted token).

Fix path (NOT relaxing detect_agent — that would revive the original #45
pre-P0-1 bug where [Claude #N] citations inside Codex bodies caused
mis-detection):

1. AUDIT_ONLY_NOTE updated to enforce comment format:
   - FIRST non-empty line MUST be `[Claude #N] <stage>` or `[Codex #N] <stage>`
   - Audit anchor / banners / prefaces MUST appear line 2 or later
   - Concrete CORRECT example included
   - Explicit warning that violation breaks stage advance

2. is_codex None guard auto-supplements:
   - When _audit_mode(title) AND detect_agent returns None, orchestrator
     posts a Gitea supplement comment requesting the correct format
   - Next round's Claude/Codex see the supplement and correct
   - Breaks the infinite loop automatically (no manual ctrl-C needed)

3. Regression tests in TestDetectAgent (test_orchestrator_core.py):
   - test_audit_anchor_preface_breaks_detection: confirms P0-1 strict
     correctly returns None when anchor is first line
   - test_audit_anchor_after_header_works: correct format passes

Total: 96/96 pytest pass (94 prior + 2 P5 regression).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

tests/orchestrator_unit/test_orchestrator_core.py

@@ -97,6 +97,37 @@ Addressing [Codex #2] findings ...
         assert detect_agent("[Codex#1] hello") == "codex"
         assert detect_agent("[Claude#5] hi") == "claude"
 
+    def test_audit_anchor_preface_breaks_detection(self):
+        """P5 (2026-05-20) — regression: AUDIT-ONLY mode 의 'Audit anchor:' preface 가
+        첫 줄에 박히면 detect_agent 는 None 반환 (P0-1 strict 의도된 동작).
+        이게 #56 (INTEGRATION-AUDIT-02) 의 Stage 4 Round #14 infinite loop 의 직접 원인.
+        해결책 = detect_agent 완화 X, AUDIT_ONLY_NOTE 가 agent header 를 first line 으로 강제."""
+        body_anchor_first = (
+            "Audit anchor: This audit verifies pipeline contracts...\n"
+            "It does not implement runtime code.\n"
+            "\n"
+            "[Codex #14] Stage 4 (test-verify) Round #14 - INTEGRATION-AUDIT-02\n"
+            "\n"
+            "Verdict: PASS. Stage 3 satisfies all criteria.\n"
+            "FINAL_CONSENSUS: YES\n"
+        )
+        assert detect_agent(body_anchor_first) is None, (
+            "audit anchor preface as first line MUST cause detect_agent None "
+            "(P0-1 strict). Fix path: comment format, not detect_agent."
+        )
+
+    def test_audit_anchor_after_header_works(self):
+        """P5 (2026-05-20) — 올바른 format: agent header first line, anchor line 2+."""
+        body_header_first = (
+            "[Codex #14] Stage 4 (test-verify) Round #14 - INTEGRATION-AUDIT-02\n"
+            "\n"
+            "Audit anchor: This audit verifies pipeline contracts...\n"
+            "\n"
+            "Verdict: PASS.\n"
+            "FINAL_CONSENSUS: YES\n"
+        )
+        assert detect_agent(body_header_first) == "codex"
+
 
 # ─────────────────────────────────────────────────────────────────
 # parse_consensus — YES/NO + rewind_target