Support opencode slash commands (/compact, /model, etc.) from session UI

[bencered]

The session input sends all text as raw prompts. OpenCode exposes slash commands like /compact, /model, /cost that via its API, which can be easily invoked. Just need to add a UI representation.

[ColeMurray]

i previously looked into this specific to /compact. it's a bit more nuanced as you need to intercept the text and it alters the underlying sessionIds.

onboard with adding this. let's proceed with a plan for implementation