Startup wrote and deleted a probe item to see whether the keychain works,
waking D-Bus on Linux and risking a Keychain prompt on macOS. A read of a
missing entry answering 'no entry' proves the same; if a later write is
refused anyway, set_checked already falls back to the vault (F11).
Not verified against a live Secret Service or macOS Keychain in this
environment (no secret service here; the probe correctly reports it
unavailable).
Andy Dixon
10f93271cf