#1979 S3: picker hides dead models; 'show unavailable (N)' with reasons; unprobed = unchecked #1986

Parent: #1979. Blocked by: #1985.

Copy upstream packages/app/src/components/dialog-select-model.tsx into the overlay (classified M) and add a pure module, dialog-select-model-reachability.ts. It merges the S2 statuses, hides non-ok models by default, and provides a toggle that reveals them greyed with a human reason ('not in your Bedrock allowlist', 'needs Marketplace agreement'). Fetch through utils/amicode-fetch.ts. Add i18n strings.

AC (RED first):

• bun tests on the pure module: given ok, denied, unavailable and unchecked models, only ok shows by default, and the toggle count and reasons are correct.
• Overlay typecheck and drift green.