Move the reaper's silence timeout and scan cadence into DaemonConfig
(defaulting to the existing 60s/15s constants) with a
`with_browser_liveness` builder, so tests can drive the task in
sub-second windows.
Add an integration test that runs a real daemon and asserts a silent
heartbeat-capable browser — and its sessions — get reaped, while a
legacy browser that never heartbeated is left untouched.
Co-authored-by: Cursor <cursoragent@cursor.com>
