Allow for Github Copilot CLI as the coding harness

**Motivation**
What problem does this solve? Who runs into it and when?
Problem: some people only have access to the Github Copilot CLI (all of Microsoft). I have used this for personal projects and would love to be able to use it at work
**Proposed solution**
Add it to the list of compatible coding harnesses. It's api shouldn't be too different from ClaudeCode/Codex

**Alternatives considered**
OpenCode would work too

**Additional context**
Anything else that helps — links, screenshots, prior art.

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