Add support to OpenCode

### Before filing

- [x] I searched [open and closed issues](https://github.com/block/berd/issues?q=is%3Aissue) for duplicates.
- [x] I am using a recent [release](https://github.com/block/berd/releases) and this doesn't already exist.
- [x] This is one request, not several bundled together.

### Closest existing issue

none found

### Is this new, or an improvement?

New capability — Berd can't do this at all today

### The problem, in your terms

Please, add support to OpenCode so I can use it with Berd.

### What you do today

I cant use Berd because it does not have support to OpenCode.

### What you'd like to see

I would like to add OpenCode as an option, like Codex or Claude Code.

### Why this belongs in Berd itself

Because OpenCode is widely used.

### Non-goals

N/A

### Alternatives you considered

I cant use Berd because it does not have OpenCode.

### Mockups, prior art, or other context

_No response_

Start by inspecting how existing Codex and Claude Code options are represented and integrated in Berd. Add OpenCode as an available option with equivalent usable support, then verify that it can be selected and used from the desktop app.

Written by the indexing model from the issue text.