Add --session flag to attach command to continue session via already running headless server #3165

I would like the cli's attach command to support --session flag to allow attaching tui instance with the specified session loading.

I tried to open this as a tiny PR but got rejected for not being a collaborator, so I'm trying to open this issue instead 🙏

https://github.com/sst/opencode/compare/sst:dev...ndraiman:ndraiman/tui-attach-session?expand=1

Add --session/-s flag to the attach command to allow continuing existing sessions.
The session ID is passed through to the TUI binary when specified.