Migrate from the terminal: pick a repository and its branch, put a crew on it, watch the job #3333

Follow-up to #3264 (pull #3289), from @sentinel's review: the web /migrate connects an origin, picks a repository, picks the branch the app lives on and puts a crew on it; the terminal (src/tui) has no migrate at all.

Done:

1. A migrate screen in the TUI reachable from the dashboard: your connected origins' repositories (filterable, private marked, size shown), or a pasted URL.
2. Selecting a repository lists its branches with their last commit (origins/work-branch.ts cachedRemoteTips), the person's newest preselected (pickWorkBranch).
3. Owner, name, visibility, agent and autonomy as on the web; the action is the same migrateRepo with branch and crew - no second path.
4. The job is watched like screens/mirror.ts watches a mirror: stage checklist, detail, Esc hides and never cancels, done lands on the repository.

Test: a TUI test drives the screen against the fake GitHub from test/migrate.test.ts and ends with the repo on the chosen branch and the crew's brief naming it.

Parked by the crew: the run ended without a pull: declined: this issue builds on work that isn't on main (070ed87a), so I stopped before writing anything.