feat(core): add Vercel AI Gateway sign-in - #3

Issue for this PR

Split from anomalyco#59456 (part 2 of 3).

Depends on anomalyco#59729 (fork review: #2). This draft targets the prompts feature branch so only the Vercel change appears in the diff; upstream submission is queued until anomalyco#59729 merges.

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Builds on the integration setup prompts from part 1. Adds Vercel device sign-in, team selection, key naming/expiration, and confirmation before creating an AI Gateway key. Only the derived Gateway key is saved. Budget management is an optional dashboard link; manual key entry remains available.

OAuth client ID is REPLACE_ME. A production release needs an OpenCode-owned public client, Team read access, AI Gateway API-key write access, and confirmed device-grant support from Vercel. No borrowed client ID is included. Key replacement is part 3.

How did you verify your code works?

• Automated checks on this branch: 31 core tests passed, including four Vercel setup cases using a local HTTP server and explicit test client IDs.
• Full repository bun run check passed.

Manual verification covered the combined implementation: @pcomans tested setup and model availability on macOS before the split and before the temporary OAuth client ID was removed. This split PR has not been manually tested end to end.

Visual

<svg xmlns="http://www.w3.org/2000/svg" width="240" height="72" viewBox="0 0 240 72"><style>@media(prefers-reduced-motion:no-preference){.done{animation:pulse 3s infinite}}@keyframes pulse{0%,100%{opacity:.4}50%{opacity:1}}</style><g fill="none" stroke="currentColor" stroke-width="1.5"><rect x="4" y="14" width="62" height="44" rx="7"/><path d="M76 36h24m-6-6 6 6-6 6"/><rect x="108" y="14" width="62" height="44" rx="7"/><path d="M180 36h18m-6-6 6 6-6 6"/><circle cx="218" cy="36" r="16" opacity=".4"/><path class="done" d="m210 36 6 6 11-13"/></g><g fill="currentColor" font-family="sans-serif" font-size="9" text-anchor="middle"><text x="35" y="39">Vercel</text><text x="139" y="39">Create key</text></g></svg>

Screenshots / recordings

Setup screenshot gallery (sample data).

Checklist

• Automated tests and checks passed locally on this branch
• I have not included unrelated changes in this PR