[FEATURE]: opentui: Replace subcommands attach, spawn, thread with flags #3607

Feature hasn't been suggested before.

• I have verified this feature I'm about to request hasn't been suggested before.

Describe the enhancement you want to request

In the current design, the spawn and attach subcommands are lacking in options compared to the main opencode command (currently defined in thread.ts). I think it would make more sense to have a single command with all those variants exposed as CLI flag(s).

I personally use all those variants during development depending on the use case, and I would like to have all the options available for each of them.

I am aware that the attach command is also in mainline OC. We can keep that command for backward compat.

The following issue would be addressed by this: #3188
We can implement it only in the opentui branch, since opentui should become mainline soon.

I volunteer to implement.