fix(github): respect environment proxy in client transport

[madneal]

Problem

The GitHub search client creates a custom http.Transport with a TLS configuration, but leaves Transport.Proxy unset. A custom transport does not inherit Go's default ProxyFromEnvironment, so HTTP_PROXY and HTTPS_PROXY configured for the scanner are bypassed and GitHub requests connect directly.

Solution

• Clone http.DefaultTransport so the standard proxy behavior and transport defaults are preserved.
• Keep the existing InsecureSkipVerify setting because the deployment requires it.
• Add a regression test that verifies the GitHub transport preserves the default proxy function.

SearchCode's separate upstream 404 response is not changed by this PR.

Verification

• GO111MODULE=on go test ./search/githubsearch -run 'TestGithubHTTPTransportPreservesDefaultProxy|TestSearchCodeByOpt' -count=1
• GO111MODULE=on go test -short ./source ./service ./router
• git diff --check