* fix(embed): clear stale daemon on port before starting new one (#843)
When `uvx hindsight-embed@latest` resolves to a new version, the old
daemon may still be bound to the port, causing EADDRINUSE. Before
starting a daemon, check if the port is occupied, verify it's a
hindsight process via /health, and SIGTERM it if so.
* chore: remove unused signal import from test
* refactor: use cross-platform port check instead of lsof-only
Use socket for port check (works on all platforms), extract PID lookup
into a helper with Windows (netstat) and Unix (lsof) paths, and
extract kill logic into a testable static method.
* refactor: reuse cross-platform helpers in stop() and stop_ui()
7d6c570a3a