feat(search): replace retired Searchcode with Sourcegraph global search

[madneal]

What changed

• Replace the retired Searchcode global endpoint with Sourcegraph's Stream API.
• Search with count:all across Sourcegraph-indexed repositories, explicitly including forks and archived repositories.
• Keep existing searchcode rules compatible; no database migration is required.
• Add sourcegraph URL/token configuration and rule/provider UI labels.
• Record upstream partial-result warnings in scan logs.

Scope and limitation

This covers all repositories indexed by the configured Sourcegraph instance (public sourcegraph.com by default), alongside the existing GitHub and GitLab providers. No third-party index can guarantee every public repository; unindexed repositories, search timeouts, and upstream result limits are reported as partial coverage.

Verification

• GO111MODULE=on go test ./search/sourcegraphsearch ./search
• GO111MODULE=on go build ./...