Add GitHub app client and inline server config

[hsyntax]

Summary

• add the GitHub app API client service and its unit coverage
• inline server config reads as file-level Config definitions instead of custom config service layers
• replace the old shared server config module with direct config-provider usage in auth, webhook, and test code

Testing

• pnpm --filter @brrr/server build
• pnpm --filter @brrr/server test
• pnpm --filter @brrr/server lint
• pnpm --filter @brrr/server fmt:check
• pnpm --filter @brrr/server test:integration
• just check
• just integration