feat(tui): add optimize prompt workflow

[wangxumarshall]

### Issue for this PR

Closes #

### Type of change

- [ ] Bug fix
- [x] New feature
- [ ] Refactor / code improvement
- [ ] Documentation

### What does this PR do?

This PR adds the `optimize` feature to the TUI.

Users can now optimize the prompt they are writing before sending it. The feature includes:

- an `optimize` action in the prompt UI
- a `/optimize` slash command
- a `Ctrl+O` keyboard shortcut
- a loading state while optimization is running
- an editable review dialog before applying the optimized prompt

On the backend, the optimize flow sends the current prompt and optional recent session context to the server, selects an available model, rewrites the prompt into a clearer and more actionable version, and keeps the output in the same language as the input.

This PR also includes the plugin-facing pieces needed for the feature:
- `tui.ui.interact` support
- a sample `prompt-optimizer` plugin
- config to load that plugin from `.opencode/opencode.jsonc`

Why this works:
the feature now has a complete flow from the TUI interaction, to the server optimization request, to the returned optimized prompt, and back into the composer with a confirmation/edit step before replacing the original text.

### How did you verify your code works?

- Ran `bun test test/optimize/ui-interact.test.ts` in `packages/opencode`
- Ran `bun typecheck` in `packages/plugin`
- Ran `bun run build` in `packages/opencode`

### Screenshots / recordings

_Not included._

### Checklist

- [x] I have tested my changes locally
- [x] I have not included unrelated changes in this PR