feat(mergechecker): add GitHub MergeChecker implementation with multi-host routing

[behinddwalls]

Summary

Implements a GitHub-specific MergeChecker that uses the GraphQL API to
validate PR mergeability and stack ordering. Adds a MultiChecker router
that dispatches by Change.Source for multi-host support.

• entity/github: ChangeID parser for {scheme}://{org}/{repo}/pull/{pr}/{sha}
• extension/mergechecker/github: GraphQL-based checker with stack validation
• extension/mergechecker: MultiChecker for source-based routing
• extension/mergechecker/mock: gomock mock for testing
• orchestrator/controller/request: wire merge check into pipeline
• example/server: configure GitHub checker via env vars

Test Plan

make build && make test