agent-credential-broker: fork of vaultwarden-agent-broker + OpenBao backend

Mechanical rename (package agent_credential_broker; entry points
agent-broker{,-mcp,-plugin}; env AGENT_BROKER_*; paths agent-broker;
agent-browser credential provider 'broker'; MCP server name agent-broker).

New backend 'openbao': stdlib-only HTTP client for OpenBao/Vault KV v2
(approle login or token file/env; metadata-only ref index; lazy re-login
on 401/403; token renewal). bitwarden-cli backend kept as transition.

agent_credential_broker/audit.py

@@ -0,0 +1,41 @@
+"""Audit logging -- metadata only, never secret material."""
+
+from __future__ import annotations
+
+import json
+import threading
+from datetime import datetime, timezone
+
+from .config import audit_log_path
+
+_lock = threading.Lock()
+
+
+def _scrub(record: dict) -> dict:
+    return {k: v for k, v in record.items() if v is not None}
+
+
+def audit(operation: str, *, ref=None, adapter=None, target=None,
+          result="ok", error_code=None, detail=None):
+    entry = _scrub({
+        "ts": datetime.now(timezone.utc).isoformat(timespec="seconds"),
+        "op": operation,
+        "ref": ref,
+        "adapter": adapter,
+        "target": target,
+        "result": result,
+        "error": error_code,
+        "detail": detail,
+    })
+    line = json.dumps(entry, sort_keys=True)
+    # Belt-and-suspenders: audit lines must never carry secret material.
+    for banned in ("password", "secret", "token", "key"):
+        if isinstance(detail, str) and banned in detail.lower():
+            entry["detail"] = "REDACTED_DETAIL"
+            line = json.dumps(_scrub(entry), sort_keys=True)
+            break
+    try:
+        with _lock, open(audit_log_path(), "a") as fh:
+            fh.write(line + "\n")
+    except OSError:
+        pass  # audit must never break operations