feat: tool call audit log

Every orchestrator tool invocation is recorded to home/{user}/tool_audit/YYYY-MM-DD.jsonl.
Each entry captures: timestamp, user, tool, args (truncated), status (ok/error/denied),
result length, and a 300-char result snippet.

- tool_audit.py: JSONL writer with per-file asyncio locks; read_recent / read_recent_all_users helpers
- tools/__init__.py: hook in call_tool() — fire-and-forget record on every dispatch
- routers/audit.py: GET /api/audit/recent and /api/audit/stats (admin-only)
- tools/files.py: add home_root() to file_read allowed roots so agents can read audit JSONL

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

cortex/routers/audit.py

@@ -0,0 +1,83 @@
+"""
+Tool audit log endpoints — admin only.
+
+  GET /api/audit/recent?user=scott&days=7&limit=200
+      Returns recent tool call entries for one user (or all users if user omitted).
+
+  GET /api/audit/stats?user=scott&days=7
+      Returns aggregate counts by tool and status.
+"""
+import jwt
+from collections import Counter
+from fastapi import APIRouter, HTTPException, Query, Request
+
+from auth_utils import COOKIE_NAME, decode_token, get_user_role
+import tool_audit
+from persona import list_users
+
+router = APIRouter(prefix="/api/audit")
+
+
+def _require_admin(request: Request) -> str:
+    token = request.cookies.get(COOKIE_NAME)
+    if not token:
+        raise HTTPException(status_code=401, detail="Not authenticated")
+    try:
+        username = decode_token(token)
+    except jwt.InvalidTokenError:
+        raise HTTPException(status_code=401, detail="Invalid session")
+    if get_user_role(username) != "admin":
+        raise HTTPException(status_code=403, detail="Admin access required")
+    return username
+
+
+@router.get("/recent")
+async def audit_recent(
+    request: Request,
+    user: str = Query(None, description="Username to filter (omit for all users)"),
+    days: int = Query(7,   ge=1, le=90),
+    limit: int = Query(200, ge=1, le=1000),
+) -> dict:
+    _require_admin(request)
+
+    if user:
+        if user not in list_users():
+            raise HTTPException(status_code=404, detail=f"User not found: {user}")
+        entries = tool_audit.read_recent(user, days=days, limit=limit)
+    else:
+        entries = tool_audit.read_recent_all_users(days=days, limit=limit)
+
+    return {"entries": entries, "count": len(entries), "days": days}
+
+
+@router.get("/stats")
+async def audit_stats(
+    request: Request,
+    user: str = Query(None),
+    days: int = Query(7, ge=1, le=90),
+) -> dict:
+    _require_admin(request)
+
+    if user:
+        if user not in list_users():
+            raise HTTPException(status_code=404, detail=f"User not found: {user}")
+        entries = tool_audit.read_recent(user, days=days, limit=10000)
+    else:
+        entries = tool_audit.read_recent_all_users(days=days, limit=10000)
+
+    tool_counts: Counter = Counter()
+    status_counts: Counter = Counter()
+    user_counts: Counter = Counter()
+
+    for e in entries:
+        tool_counts[e.get("tool", "?")] += 1
+        status_counts[e.get("status", "?")] += 1
+        user_counts[e.get("user", "?")] += 1
+
+    return {
+        "total": len(entries),
+        "days": days,
+        "by_tool":   dict(tool_counts.most_common()),
+        "by_status": dict(status_counts),
+        "by_user":   dict(user_counts.most_common()),
+    }