fix: serialize decision ledger writes

harness/tht/decisions.py

@@ -1,3 +1,4 @@
+import fcntl
 import os
 import tempfile
 from datetime import UTC, datetime
@@ -7,6 +8,7 @@ from typing import Literal
 from pydantic import BaseModel
 
 DECISIONS_FILE = "review_decisions.jsonl"
+DECISIONS_LOCK_FILE = ".review_decisions.lock"
 
 # 22 tipi di the reference implementation (verified leggendo session/decisions.py) + 1 nuovo (D15):
 # `decision_retracted` per il rollback a granularità step (ritira una decisione
@@ -121,6 +123,22 @@ def append_decisions(
     if not inputs:
         return []
 
+    session_dir.mkdir(parents=True, exist_ok=True)
+    lock_path = session_dir / DECISIONS_LOCK_FILE
+    with lock_path.open("a+") as lock:
+        fcntl.flock(lock, fcntl.LOCK_EX)
+        try:
+            return _append_decisions_locked(session_dir, inputs)
+        finally:
+            fcntl.flock(lock, fcntl.LOCK_UN)
+
+
+def _append_decisions_locked(
+    session_dir: Path,
+    inputs: list[DecisionInput],
+) -> list[DecisionRecord]:
+    """Append while the caller holds the session's cross-process ledger lock."""
+
     # Fase corrente PRIMA dell'append (high-water-mark D15). Import lazy: phase.py
     # importa decisions.py (ciclo). Per i marker di fase (subject "phase:N") il valore
     # e' ridondante col subject; per le decisioni sostanziali con subject "a nome"