feat: add APort Go SDK and framework middleware

[sureshchouksey8]

Summary

• Add a Go module under sdk/go with an APort policy verification client.
• Support configurable base URL, injected HTTP client, bearer auth, structured verify responses, and fail-closed RequirePolicy behavior.
• Add middleware adapters for net/http, Gin, Echo, and Fiber that read X-Agent-ID, build request context, block denied policy decisions, and expose successful decisions in request/framework context.
• Include a runnable net/http refund guard example and unit tests for allow and deny paths.

Verification

• git diff --check
• Reviewed Go source for gofmt-style formatting.
• Go toolchain is not installed in this Codex environment, so I could not run go test ./... locally.

/claim #17

Closes #17

[sureshchouksey8]

This is ready for review. The Go SDK PR includes the core verification client, fail-closed RequirePolicy, net/http + Gin/Echo/Fiber middleware, unit tests, README, and a runnable net/http example.