FN-5714: reject linking features to non-active tasks

Gracefully handle feature-to-task linking when the referenced task is archived, deleted, or missing.

- enforce active-board task validation in mission store linkage path with clear error text
- return surfaced validation errors from fn_feature_link_task instead of throwing opaque failures
- add regression coverage in core and CLI extension tests for archived/non-active task references
- add changeset and sync Fusion skill reference docs with the new fn_feature_link_task behavior

Files changed:
 .changeset/fn-5714-link-archived-task.md           |  5 +++
 .../cli/skill/fusion/references/extension-tools.md |  2 +-
 .../skill/fusion/references/fusion-capabilities.md |  2 +-
 packages/cli/src/__tests__/extension.test.ts       | 50 ++++++++++++++++++++++
 packages/cli/src/extension.ts                      | 35 +++++++++------
 packages/core/src/__tests__/mission-store.test.ts  | 17 ++++++++
 packages/core/src/mission-store.ts                 |  9 ++++
 7 files changed, 106 insertions(+), 14 deletions(-)

Fusion-Task-Id: FN-5714

Fusion-Task-Lineage: d84c45cf-0a0d-47a3-81c5-f99d16e03ecf

packages/core/src/__tests__/mission-store.test.ts

@@ -1244,6 +1244,23 @@ describe("MissionStore", () => {
       expect(taskRow.sliceId).toBe(slice.id);
     });
 
+    it("throws a clear error when linking to a task not on the active board", () => {
+      const mission = store.createMission({ title: "Mission" });
+      const milestone = store.addMilestone(mission.id, { title: "Milestone" });
+      const slice = store.addSlice(milestone.id, { title: "Slice" });
+      const feature = store.addFeature(slice.id, { title: "Linkable" });
+
+      expect(() => store.linkFeatureToTask(feature.id, "FN-ARCHIVED")).toThrow(
+        `Cannot link feature ${feature.id} to task FN-ARCHIVED: task is not on the active board (it may be archived, deleted, or never existed). Only active tasks can be linked to features.`,
+      );
+
+      const unchanged = store.getFeature(feature.id)!;
+      expect(unchanged.taskId).toBeUndefined();
+      expect(unchanged.status).toBe("defined");
+      expect(unchanged.loopState).toBe("idle");
+      expect(unchanged.implementationAttemptCount).toBe(0);
+    });
+
     it("emits feature:linked event", () => {
       createTaskInDb(db, "FN-001");
 

packages/core/src/mission-store.ts

@@ -2085,6 +2085,15 @@ export class MissionStore extends EventEmitter<MissionStoreEvents> {
       throw new Error(`Feature ${featureId} not found`);
     }
 
+    const liveTask = this.db
+      .prepare(`SELECT id FROM tasks WHERE id = ? AND "deletedAt" IS NULL`)
+      .get(taskId) as { id: string } | undefined;
+    if (!liveTask) {
+      throw new Error(
+        `Cannot link feature ${featureId} to task ${taskId}: task is not on the active board (it may be archived, deleted, or never existed). Only active tasks can be linked to features.`,
+      );
+    }
+
     const linkage = this.resolveTaskLinkage(feature.sliceId);
 
     // When first linking (loopState is idle or falsy), transition to implementing