test: add httptest-based tests for real GitHub client

[packet-mover]

Summary

• Adds client_test.go with 18 httptest-based tests covering realGitHubClient behavior: field mapping, pagination, 403/404 graceful fallthrough, 500 error propagation, ruleset aggregation, and issue creation
• Adds newTestGitHubClient(baseURL) helper in client.go for pointing go-github at a test server
• Updates AGENTS.md to document the two-layer testing approach: mock-based tests for scanner logic/rules/reports, httptest-based tests for real HTTP client behavior

This is a follow-up to #25 (go-github migration). The httptest tests cover error handling paths that the mock client cannot exercise - including the 403 handling for private repos without GitHub Pro.

Test plan

• All 60 tests pass (go test ./...)
• New tests cover: ListRepos (single page, pagination, error), GetTree (field mapping, error), GetBranchProtection (success, no reviewers, 404, 403, 500), GetRulesets (success, multiple rules, no matching rules, 404, 403, 500), CreateIssue (success, error)