fix(auth): accept direct Anthropic API keys

[EfeDurmaz16]

Summary

• accept ANTHROPIC_API_KEY as valid Claude Agent SDK credentials for direct Anthropic usage
• update .env.example and docs to recommend AI_GATEWAY_API_KEY for gateway routing and ANTHROPIC_API_KEY for direct Anthropic
• add preflight coverage so direct Anthropic users are not blocked before the SDK runs

Fixes #55.

Verification

• pnpm install --frozen-lockfile
• pnpm vitest run --project cli packages/deepsec/src/__tests__/preflight.test.ts
• pnpm lint
• pnpm --filter deepsec build
• git diff --check

Note: install completed with an existing workspace bin warning for node_modules/.bin/deepsec before the package build artifact exists.

[vercel]

@EfeDurmaz16 is attempting to deploy a commit to the Vercel Labs Team on Vercel.

A member of the Team first needs to authorize it.

[vercel]

Additional Suggestion:

resolveBrokeredCredentials() only reads ANTHROPIC_AUTH_TOKEN, ignoring ANTHROPIC_API_KEY, causing sandbox runs to silently fail with 401 when only ANTHROPIC_API_KEY is set.

[EfeDurmaz16]

Checked the review suggestion about direct ANTHROPIC_API_KEY support. The current PR tip already contains the follow-up fix.

Current branch state:

• resolveBrokeredCredentials() now falls back from ANTHROPIC_AUTH_TOKEN to ANTHROPIC_API_KEY.
• credential-brokering tests cover direct ANTHROPIC_API_KEY and precedence when both variables are set.

Verification:

• pnpm test packages/deepsec/src/__tests__/credential-brokering.test.ts packages/deepsec/src/__tests__/preflight.test.ts

Commit: 76d1381