Preserve explicit machine choice for project checkouts

[ymichael]

Human comments

What was wrong

The shared new-thread composer treated checkout-capable providers as eligible on every connected machine, even when a machine had no source for the selected project. That exposed Project checkout and Worktree on an unconfigured machine; if that explicit choice later became invalid, resolution could silently fall back to the primary machine. This regressed in b3c5434 (#3274).

What changed

• Derive existing-machine checkout eligibility from the project's sources and the existing project-scoped provider query.
• Hide Project checkout and Worktree when that project is not set up on a machine, while preserving valid local, remote, and hostless choices.
• Retain an explicit selected machine and block submission if its checkout disappears or becomes unavailable instead of redirecting the thread elsewhere.
• Add focused composer regressions for per-machine eligibility, invalidation, availability, keyboard behavior, and valid fallback-free selections.

Before	After

The paired 390×844 dark-theme captures use the actual source components with synthetic SDK and machine data. The before image is from base 1ebdc56a50; the after image is from pushed head 63920f8faa.

No server/daemon wire contract, CLI, public SDK, generated file, or lockfile changes.

How you verified

• New eligibility regressions failed before the fix and passed after it.
• pnpm exec turbo run build typecheck test --filter=@bb/app --force — build and typecheck passed; 4,469 tests passed and 3 were intentionally skipped.
• Extra templates validation — 44 tests passed.
• Browser target matrix — 20 submission/control cases plus four rejection, keyboard, focus, root, and overflow cases passed at 1280×900 and 390×844 in light and dark themes.
• git diff --check origin/main...HEAD passed, and Git reports a conflict-free merge with current main.

AGENT GENERATED