[Feature]: Opencode provider

[brrock]

Before submitting

• I searched existing issues and did not find a duplicate.
• I am describing a concrete problem or use case, not just a vague idea.

Area

apps/server

Problem or use case

I just want to use my opencode models in t3code

Proposed solution

Opencode provider

Why this matters

My copilot free trial is running out lol

Smallest useful scope

Simple opencode provider. Could be out of this project scope

Alternatives considered

No response

Risks or tradeoffs

No response

Examples or references

No response

Contribution

• I would be open to helping implement this.