fix(#85): claude_session — retry על כשלים חולפים של claude -p

שורש #85 התברר: `claude -p` נכשל מדי פעם ב-exit מהיר + stderr ריק על
פרומפטים גדולים/איטיים (CEO write_interim_draft, learning_loop distillation),
**אותו פרומפט מצליח בריצה חוזרת** — כשל חולף, לא nesting (אומת: nested claude
מ-bash וגם פרומפט 70K הצליחו; הכשל אינו דטרמיניסטי).

query() עוטף spawn+communicate ב-לולאת retry (MAX_RETRIES=3, backoff לינארי
5s*attempt). FileNotFoundError + timeout נשארים דטרמיניסטיים (ללא retry).
empty-response גם מטופל כ-transient.

אומת e2e: distillation על 1130-25 רץ בהצלחה → pair=analyzed (9 שינויים,
6 style_method, 33.8% diff). פותר גם את write_interim_draft של ה-CEO.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

mcp-server/src/legal_mcp/services/claude_session.py

@@ -40,6 +40,14 @@ logger = logging.getLogger(__name__)
 DEFAULT_TIMEOUT = 1800
 LONG_TIMEOUT = 3600  # opus block writing on full case context
 
+# #85 — `claude -p` fails intermittently with a fast non-zero exit and empty
+# stderr (observed on large/slow cold prompts: CEO write_interim_draft,
+# learning_loop distillation). The SAME prompt succeeds on retry, so the bail is
+# transient — retry with linear backoff. Timeouts and "CLI not found" are
+# deterministic and are NOT retried.
+MAX_RETRIES = 3
+RETRY_BACKOFF_BASE = 5  # seconds; sleep = base * attempt_number
+
 
 async def query(
     prompt: str,
@@ -94,53 +102,69 @@ async def query(
     if effort:
         cmd += ["--effort", effort]
 
-    try:
-        proc = await asyncio.create_subprocess_exec(
-            *cmd,
-            stdin=asyncio.subprocess.PIPE,
-            stdout=asyncio.subprocess.PIPE,
-            stderr=asyncio.subprocess.PIPE,
-        )
-    except FileNotFoundError:
-        raise RuntimeError(
-            "Claude CLI not found. This module only works when invoked "
-            "from the local MCP server — see the architectural rule in "
-            "the module docstring. If this error came from a FastAPI "
-            "endpoint in the container, refactor the call into an MCP "
-            "tool that the chair triggers from Claude Code."
-        )
+    size_info = f"; prompt_len={len(full_prompt):,} chars" if len(full_prompt) > 100_000 else ""
+    last_err = "unknown error"
 
-    try:
-        stdout_b, stderr_b = await asyncio.wait_for(
-            proc.communicate(input=full_prompt.encode("utf-8")),
-            timeout=timeout,
-        )
-    except asyncio.TimeoutError:
-        # wait_for cancellation alone leaves the child running.
+    for attempt in range(1, MAX_RETRIES + 1):
         try:
-            proc.kill()
-            await proc.wait()
-        except ProcessLookupError:
-            pass
-        raise RuntimeError(f"Claude CLI timed out after {timeout}s")
+            proc = await asyncio.create_subprocess_exec(
+                *cmd,
+                stdin=asyncio.subprocess.PIPE,
+                stdout=asyncio.subprocess.PIPE,
+                stderr=asyncio.subprocess.PIPE,
+            )
+        except FileNotFoundError:
+            # Deterministic — never retry.
+            raise RuntimeError(
+                "Claude CLI not found. This module only works when invoked "
+                "from the local MCP server — see the architectural rule in "
+                "the module docstring. If this error came from a FastAPI "
+                "endpoint in the container, refactor the call into an MCP "
+                "tool that the chair triggers from Claude Code."
+            )
 
-    if proc.returncode != 0:
-        stderr = stderr_b.decode("utf-8", errors="replace").strip()[:500] or "unknown error"
-        size_info = f"; prompt_len={len(full_prompt):,} chars" if len(full_prompt) > 100_000 else ""
-        raise RuntimeError(f"Claude CLI failed (exit {proc.returncode}): {stderr}{size_info}")
+        try:
+            stdout_b, stderr_b = await asyncio.wait_for(
+                proc.communicate(input=full_prompt.encode("utf-8")),
+                timeout=timeout,
+            )
+        except asyncio.TimeoutError:
+            # wait_for cancellation alone leaves the child running. A timeout is
+            # a real ceiling, not a transient blip — don't retry.
+            try:
+                proc.kill()
+                await proc.wait()
+            except ProcessLookupError:
+                pass
+            raise RuntimeError(f"Claude CLI timed out after {timeout}s")
 
-    stdout = stdout_b.decode("utf-8", errors="replace").strip()
-    if not stdout:
-        raise RuntimeError("Claude CLI returned empty response")
+        if proc.returncode != 0:
+            stderr = stderr_b.decode("utf-8", errors="replace").strip()[:500] or "unknown error"
+            last_err = f"exit {proc.returncode}: {stderr}"
+        else:
+            stdout = stdout_b.decode("utf-8", errors="replace").strip()
+            if stdout:
+                # claude -p --output-format json returns {"type":"result","result":"..."}
+                try:
+                    data = json.loads(stdout)
+                    if isinstance(data, dict) and "result" in data:
+                        return data["result"]
+                    return stdout
+                except json.JSONDecodeError:
+                    return stdout
+            last_err = "empty response"
 
-    # claude -p --output-format json returns {"type":"result","result":"..."}
-    try:
-        data = json.loads(stdout)
-        if isinstance(data, dict) and "result" in data:
-            return data["result"]
-        return stdout
-    except json.JSONDecodeError:
-        return stdout
+        # Transient failure — retry with linear backoff unless this was the last try.
+        if attempt < MAX_RETRIES:
+            logger.warning(
+                "claude -p attempt %d/%d failed (%s%s) — retrying in %ds",
+                attempt, MAX_RETRIES, last_err, size_info, RETRY_BACKOFF_BASE * attempt,
+            )
+            await asyncio.sleep(RETRY_BACKOFF_BASE * attempt)
+
+    raise RuntimeError(
+        f"Claude CLI failed after {MAX_RETRIES} attempts ({last_err}){size_info}"
+    )
 
 
 async def query_json(