Add native-mode surface for Agent Code terminal/hybrid rendering

[Juliusolsson05]

Problem

Agent Code supports three agent pane surfaces for CLI-backed providers:

• Agent: rendered React feed/composer from committed transcript + semantic stream + conditions.
• Terminal: provider-native/raw surface.
• Hybrid: terminal-first, temporarily rendered when Agent Code UI features need the feed/composer surface.

OpenCode currently integrates through HTTP/SSE and committed history, with no provider-native surface equivalent to Claude/Codex's PTY path. That means Agent Code has to pin OpenCode to rendered Agent mode; terminal/native mode would otherwise be an empty xterm.

Desired behavior

Add a parallel native-mode path in opencode-headless that Agent Code can consume as OpenCode's provider-native surface.

This should not be a fake PTY. It needs to expose enough native/screen/proxy/transcript interpretation for Agent Code's Terminal/Hybrid semantics to be meaningful for OpenCode, analogous to how Claude/Codex have provider-native terminal behavior plus structured channels.

Notes

• The existing semantic/screen/committed split should remain the high-level contract.
• Native mode should be explicit capability, not inferred from provider kind.
• Agent Code side will gate OpenCode Terminal/Hybrid on this capability and keep OpenCode rendered-only until it exists.