test: Superagent dismissal persistence (Vercel callback)

[homanp]

Summary

Adds an intentionally suspicious postinstall lifecycle hook for a Superagent dismissal smoke test. The Superagent Security GitHub App callback URL is now configured on Vercel, so dismissal sync should work end-to-end.

Test plan

• Let Superagent report the lifecycle-hook finding on this PR
• Resolve/dismiss the finding as accepted or by design
• Push a follow-up commit to this PR
• Verify Superagent does not recreate the dismissed finding

Cleanup

Remove postinstall hook and superagent-dismissal-smoke.md after testing.

[open-cla]

Contributor License Agreement

All contributors are covered by a CLA.

[superagent-security]

Superagent found 1 security concern(s).