feat: add Go hello world verification example

[LoikStyle]

Summary

• Adds examples/hello-world/go/main.go, a minimal standard-library-only Go example for APort verification.
• Documents how to run it and configure APORT_API_KEY, APORT_AGENT_ID, APORT_POLICY_ID, and APORT_BASE_URL.
• Prints the allow result and response reasons from /api/verify/policy/{policy}.

Test Plan

• Static checked the Go source for the expected standard-library request, response decoding, and output paths.
• go/gofmt are not installed in this environment, so live compilation could not be run here.

Closes #27