Protect dashboard OAuth credentials with the same file-safety guarantees as other auth paths

The web dashboard's Anthropic OAuth helper wrote the credential file
straight to its final destination and relied on the process umask for
permissions. That left the dashboard-specific path weaker than the
existing auth writers, which already use owner-only permissions and
safer write semantics.

This change keeps the scope narrow: make the dashboard helper write via
a temp file + replace, chmod the final file to owner-only, and add a
focused regression test for both permission handling and atomic-write
behavior.

Constraint: Must preserve the existing dashboard OAuth flow and credential-pool side effects
Rejected: Broader auth-storage refactor | unnecessary scope for a single verified inconsistency
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Keep dashboard credential writes aligned with existing auth storage semantics; do not reintroduce direct write_text() here without matching chmod/atomic behavior
Tested: pytest -o addopts='' tests/hermes_cli/test_web_server_oauth_write.py tests/hermes_cli/test_web_server.py -q (78 passed)
Not-tested: Cross-platform permission semantics on Windows-managed filesystems

hermes_cli/web_server.py

@@ -16,6 +16,7 @@ import json
 import logging
 import os
 import secrets
+import stat
 import subprocess
 import sys
 import threading
@@ -1686,7 +1687,25 @@ def _save_anthropic_oauth_creds(access_token: str, refresh_token: str, expires_a
         "expiresAt": expires_at_ms,
     }
     _HERMES_OAUTH_FILE.parent.mkdir(parents=True, exist_ok=True)
-    _HERMES_OAUTH_FILE.write_text(json.dumps(payload, indent=2), encoding="utf-8")
+    tmp_path = _HERMES_OAUTH_FILE.with_name(
+        f"{_HERMES_OAUTH_FILE.name}.tmp.{os.getpid()}.{secrets.token_hex(8)}"
+    )
+    try:
+        with tmp_path.open("w", encoding="utf-8") as handle:
+            handle.write(json.dumps(payload, indent=2))
+            handle.flush()
+            os.fsync(handle.fileno())
+        os.replace(tmp_path, _HERMES_OAUTH_FILE)
+        try:
+            _HERMES_OAUTH_FILE.chmod(stat.S_IRUSR | stat.S_IWUSR)
+        except OSError:
+            pass
+    finally:
+        try:
+            if tmp_path.exists():
+                tmp_path.unlink()
+        except OSError:
+            pass
     # Best-effort credential-pool insert. Failure here doesn't invalidate
     # the file write — pool registration only matters for the rotation
     # strategy, not for runtime credential resolution.