fix: serve Ctrl-C stop

[zkd-11]

Summary

This PR fixes shutdown handling for the kage serve command:

• Handles both SIGINT and SIGTERM so the running server can be stopped cleanly.
• Shuts down the HTTP server on cancellation so the listening port is released.

Details

Previously, kage serve could print press Ctrl-C to stop, but pressing Ctrl+C did not reliably terminate the process in some terminal environments. The process could keep running and continue holding port 8800.

This change updates the CLI entrypoint to listen for both os.Interrupt and syscall.SIGTERM. Once the first signal is received, it restores the default signal behavior so a second Ctrl+C can force termination.

The serve command now shuts down its HTTP server through http.Server.Shutdown with a short timeout, falling back to Close if graceful shutdown fails. This releases the listening port after cancellation.

Testing

go test ./...
go build -o ./bin/kage ./cmd/kage
./bin/kage serve .

Manual checks:

• Ctrl+C stops kage serve.
• Port 8800 is released after shutdown.
• Sending SIGTERM also stops the server cleanly.