Behind PI_EXPERIMENTAL=1, show a first-time setup dialog on interactive startup when the default agent directory is used and settings.json does not exist. The dialog preselects the detected terminal appearance with an explicit dark/light choice (live preview) and asks for opt-in analytics data sharing. Submitting writes settings.json, which serves as the completion marker; opting in stores a generated trackingId. Escape at any point skips out of setup.
Signal-triggered shutdown (SIGTERM/SIGHUP) now emits session_shutdown
before any terminal writes, and SIGHUP no longer hard-exits, so extension
resources (e.g. sockets) are released and the terminal is restored even
when the terminal is gone. A genuinely dead terminal still exits without
hot-spinning via the EIO error path.
closes#5080