ci: publish formula under Formula directory

[0xRaghu]

Summary

• set GoReleaser Homebrew formula directory to Formula
• update the release verifier to read Formula/mnemo.rb through the GitHub API
• verify the generated binary formula version and release-asset URLs instead of the old source-tarball URL

Why

The v1.3.6 release created GitHub release assets successfully, but GoReleaser wrote mnemo.rb at the tap root. The existing Formula/mnemo.rb therefore stayed on v1.3.4 and the verifier correctly failed.

Verification

• workflow YAML parse
• go run github.com/goreleaser/goreleaser/v2@v2.9.0 check
• go test ./...