feat(dashboard-auth): json-lines audit log at $HERMES_HOME/logs/dashboard-auth.log

Phase 1, Task 1.4. Records every auth event (login start/success/failure,
logout, refresh success/failure, revoke, session verify failure, WS
ticket mint) as one JSON object per line. Token-like kwargs (access_token,
refresh_token, code, code_verifier, state, ticket, cookie, Authorization)
are dropped before serialisation so the log never contains live secrets.

Write failures log at WARNING but never raise — auth flows must not fail
because the audit logger broke.

hermes_cli/dashboard_auth/audit.py

@@ -0,0 +1,86 @@
+"""Audit log for dashboard-auth events.
+
+Profile-aware location: ``$HERMES_HOME/logs/dashboard-auth.log``.
+Format: one JSON object per line. Token-like fields are stripped before
+serialisation to avoid leaking refresh tokens or JWTs to disk.
+
+This module deliberately keeps a minimal dependency surface — no imports
+from ``hermes_constants`` or other hermes_cli modules — so it can be
+imported safely from middleware code that loads early in the startup
+sequence.
+"""
+from __future__ import annotations
+
+import datetime as _dt
+import enum
+import json
+import logging
+import os
+import threading
+from pathlib import Path
+from typing import Any
+
+_log = logging.getLogger(__name__)
+_write_lock = threading.Lock()
+
+# Field names that must never appear in the log raw. Any kwarg matching
+# these is silently dropped.
+_REDACTED_FIELDS: frozenset = frozenset({
+    "access_token", "refresh_token", "code", "code_verifier",
+    "state", "ticket", "cookie", "Authorization", "authorization",
+})
+
+
+class AuditEvent(enum.Enum):
+    """Event types written to dashboard-auth.log.
+
+    Values are the literal ``event`` field on the JSON line.
+    """
+
+    LOGIN_START = "login_start"
+    LOGIN_SUCCESS = "login_success"
+    LOGIN_FAILURE = "login_failure"
+    LOGOUT = "logout"
+    REFRESH_SUCCESS = "refresh_success"
+    REFRESH_FAILURE = "refresh_failure"
+    REVOKE = "revoke"
+    SESSION_VERIFY_FAILURE = "session_verify_failure"
+    WS_TICKET_MINTED = "ws_ticket_minted"
+
+
+def _resolve_log_path() -> Path:
+    """``$HERMES_HOME/logs/dashboard-auth.log`` with the standard fallback.
+
+    Mirrors ``hermes_constants.get_hermes_home`` semantics: env var wins,
+    else ``~/.hermes``. A local copy avoids an import cycle with the
+    middleware which lives below ``hermes_cli``.
+    """
+    home = os.environ.get("HERMES_HOME") or str(Path.home() / ".hermes")
+    return Path(home) / "logs" / "dashboard-auth.log"
+
+
+def audit_log(event: AuditEvent, **fields: Any) -> None:
+    """Append one event to the audit log.
+
+    Token-like fields are dropped. Missing log directory is created.
+    Write failures are logged at WARNING but never raise — auth must not
+    fail because the audit logger broke.
+    """
+    safe_fields = {
+        k: v for k, v in fields.items()
+        if k not in _REDACTED_FIELDS
+    }
+    entry = {
+        "ts": _dt.datetime.now(_dt.timezone.utc).isoformat(),
+        "event": event.value,
+        **safe_fields,
+    }
+    line = json.dumps(entry, separators=(",", ":")) + "\n"
+    path = _resolve_log_path()
+    try:
+        path.parent.mkdir(parents=True, exist_ok=True)
+        with _write_lock:
+            with open(path, "a", encoding="utf-8") as f:
+                f.write(line)
+    except Exception as e:
+        _log.warning("dashboard-auth audit log write failed: %s", e)