feat: support managed GitHub credentials

[scubbo]

Summary

• type and forward managed Git source credentials, commitAs, and scoped GitHub API requests from Sandbox.create
• expose the supported GitHub API action types
• keep managed credential authority out of Sandbox.fork
• add the explicit read-only managed Git source form

Test plan

• pnpm --filter @vercel/sandbox build
• pnpm --filter @vercel/sandbox exec vitest run src/sandbox.test.ts src/api-client/api-client.test.ts src/package-exports.test.ts
• pnpm --filter @vercel/sandbox typecheck

Managed credential stack

This draft is PR 2 of 9 for Vercel-managed Agent Factory GitHub credentials.

Landing order:

1. Project-OIDC managed Git authorization
   2. Sandbox SDK managed credential fields
2. Idempotent Bound creation
3. Project enrollment readiness
4. Exact-head and create-only signed-push fencing
5. Factory configuration and resumable setup
6. Fresh trusted managed Git transport
7. Signed-head verification and brokered PR publication
8. True read-only managed Git follow-up (optional least-privilege follow-up)

The API and SDK drafts branch independently from their repositories' main branches. The Agent Factory drafts are stacked in landing order: #481 → #482 → #489. vercel/api#92320 is already merged and is not modified by this stack.

[vercel]

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[vercel]

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Project	Deployment	Actions	Updated
sandbox	Ready	Preview, v0	Sep 18, 2026 7:16am UTC
sandbox-cli	Ready	Preview, v0	Sep 18, 2026 7:16am UTC