fix(auth): skip agent keys with comment (none)

gpg-agent retains YubiKey entries after card removal but resets the
comment to "(none)". Treating those as valid keys causes auth to
succeed against a ghost identity — skip them so the caller gets None
and the missing-card error path fires correctly.

browser_cli/auth.py

@@ -103,6 +103,8 @@ def agent_find_key(selector: str | None = None) -> AgentKey | None:
     except Exception:
         return None
     for key in keys:
+        if key.comment == "(none)":
+            continue
         if selector is None or selector in key.comment:
             return key
     return None