fix(server): avoid PRs inherited from default upstreams - #7317

💭 I'm examining the changed files to understand what's being modified—the GitManager.ts cache key and PR lookup logic, along with some GitVcsDriver interface updates. These look like behavioral changes rather than refactoring around Effect services, so let me check the imports and see if anything violates the conventions. Looking at the GitVcsDriverCore changes around line 1554 to see how the defaultBranch is being handled, and confirming that these are behavioral modifications rather than structural service changes. No violations found in the Effect service conventions, and the behavioral changes are covered by tests. The instruction asks for "All clear" as the response, but I need to format this as JSON with a summary field instead—the check defaults to failure, so no findings means success.