Add pi / oh-my-pi as a coding agent

[louisgundelwein]

Add pi (oh-my-pi) as a selectable coding agent, alongside the existing agents — enable/availability/auth and running worktrees through it like the others.

Related: #473

[sbertix]

Sure. Wanna send a PR? 🙇‍♂️

[jparr721]

Looks like the PR was closed without explanation. Any chance this gets in? I can try and pick it up if needed.

[sbertix]

Asked in the PR 💪

[louisgundelwein]

@sbertix could you please mark this issue as ready? PR #580 links it with Closes #492; the policy check is now blocked solely because this feature issue has no ready label.

[github-actions]

This is marked ready, so it is open for a pull request. If you'd like to work on it, open a pull request and link it with Closes #492 in the description. If the issue is assigned to someone, it is reserved for them; otherwise anyone is welcome to pick it up.

See CONTRIBUTING.md for the full flow.