fix(daemon): self-evict when socket is taken over; safe unlink-on-close

[couragehong]

runed is meant to be a singleton on $RUNED_HOME/embedding.sock, but when the home dir is recreated out from under a running daemon (e.g. a dev rm -rf ~/.runed + reinstall) the socket path and lock files get new inodes, so the single-instance guards fail open and a second daemon spawns. The old daemon keeps running on a path that no longer points at it, and nothing reaps it -- orphan daemons accumulate. Separately, Go's UnixListener unlink-on-close lets a dying daemon delete a healthy peer's socket file that happens to occupy the same path.

• ipc.Listen disables net's unconditional unlink-on-close and records the bound socket's (dev,ino); Close removes the file only if the path still refers to our own socket, and StillOwned exposes the same check. A cross-platform Listener interface carries StillOwned to callers.
• runed runs a self-eviction watchdog: if its socket is unlinked or rebound by another daemon, it triggers the normal graceful shutdown and cancels the daemon context, exiting 0 so the supervisor stays down rather than restarting into the stolen path. Boot-failure paths return nil when evicted so an eviction during self-bootstrap also exits 0.

Adds regression tests: StillOwned transitions, and that an evicted listener's Close does not delete a socket another listener rebound.