fix(FN-5193): restore task source typing for workspace typecheck

- Add the optional Task.source field back to the core Task interface
- Preserve durable task creation and import provenance typing alongside sourceIssue metadata
- Unblock workspace typechecking for consumers that read task source metadata

Fusion-Task-Id: FN-5193

packages/core/src/types.ts

@@ -1630,6 +1630,8 @@ export interface Task {
    * Distinct from issueInfo/sourceIssue, which describe imported source issues.
    */
   githubTracking?: TaskGithubTracking;
+  /** Durable source provenance for task creation/import metadata. */
+  source?: TaskSource;
   /** Durable source provenance for the originating external issue. */
   sourceIssue?: TaskSourceIssue;
   log: TaskLogEntry[];