[skills] Document Passport identities in Vercel Connect

[dvoytenko]

Summary

• Document passing the authenticated identity returned by @vercel/passport's requireIdentity() directly as the subject for getToken().
• Include a scoped Slack example and clarify server-side usage, connector attachment, and provider authorization requirements.
• Add Passport-related intent signals and retrieval metadata to the Connect skill.

Validation

• npx --yes bun test tests/skill-map-frontmatter.test.ts tests/build-skill-map.test.ts tests/user-prompt-submit-skill-inject.test.ts — 112 tests passed.
• npx --yes bun run scripts/validate.ts — all checks passed.
• git diff --check — passed.