feat(FN-3876): add GitHub tracking test coverage across settings, issues, s

Adds comprehensive test coverage for GitHub tracking across four areas: settings inheritance, issue creation, status sync transitions, and unlink sync regression — spanning 280 lines of new tests across four test files with minimal store.ts adjustment.

Fusion-Task-Id: FN-3876

packages/core/src/store.ts

@@ -2522,6 +2522,7 @@ export class TaskStore extends EventEmitter<TaskStoreEvents> {
       priority: normalizeTaskPriority(input.priority),
       tokenUsage: input.tokenUsage,
       sourceIssue: input.sourceIssue,
+      githubTracking: input.githubTracking,
       sourceType: input.source?.sourceType ?? "unknown",
       sourceAgentId: input.source?.sourceAgentId,
       sourceRunId: input.source?.sourceRunId,