serve: add socket listener mode

[thdxr]

Summary

• Add opencode serve --socket for Unix socket paths and Windows named pipes.
• Extend Server.listen to accept and return socket while preserving TCP listener behavior.
• Add listener coverage for serving HTTP over a socket path.

Testing

• bun typecheck
• bun test test/server/httpapi-listen.test.ts -t "listens on a socket path"

[greptile-apps]

Your free trial has ended. If you'd like to continue receiving code reviews, you can add a payment method here.