Probe Github App

Prove the pasted GitHub App credentials actually work, on demand -- the same "save then verify" shape /api/system/wise/probe and /api/system/browserbase/probe already give their own cards. Mints an installation token and lists the App's visible repositories, checking for PIGEONLY/roost-v2 -- the exact proof issue #2488 asked for ("a 'test' that mints an installation token and calls GET /installation/repositories proving it sees roost-v2"). Never raises -- github_app.probe_github_app is itself non-raising and records the verdict either way; this only translates that into what the panel shows, pass/fail, never a stack trace.