When the daemon process crashes during startup (e.g., missing
Playwright), stderr was discarded via Stdio::null(), so users only
saw "Daemon failed to start (port: ...)" with no diagnostic info.
Now captures daemon stderr via Stdio::piped() and detects early process
exit with try_wait() during the startup polling loop. If the daemon
crashes, the actual error from stderr is shown to the user.
Also forwards --debug flag to the daemon process as AGENT_BROWSER_DEBUG
so debug logging works end-to-end.
Closes#56
Chris Tate
139dd0ec5a