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

Add GPT-5.4 fast mode (/fast) to the TUI.

This feature was introduced with GPT-5.4, and opencode should expose it in a first-class way.

User-facing behavior:

• /fast toggles fast mode
• the prompt status line shows fast when enabled
• supported models send the fast-mode request option

Implementation direction:

• represent fast support as model-level controls metadata
• keep support checks, UI state, and request behavior in sync
• keep legacy fast fields as compatibility aliases during migration