feat(KB-034): add task archive/unarchive functionality

- Add 'archived' column to task store with archiveTask and unarchiveTask methods
- Add CLI commands: kb task archive <id> and kb task unarchive <id>
- Add pi extension tools for archive and unarchive operations
- Add dashboard API endpoints POST /api/tasks/:id/archive and /unarchive
- Add Archived column to board UI with archive/unarchive buttons
- Prevent drag-drop into archived column, add visual distinction
- Include duplicateTask from concurrent branch in merge resolution

packages/dashboard/app/__tests__/column-fixed-width.test.ts

@@ -35,9 +35,9 @@ describe("column fixed-width CSS", () => {
   });
 
   describe("desktop .board grid template", () => {
-    it("still uses repeat(5, minmax(260px, 1fr))", () => {
+    it("uses repeat(6, minmax(260px, 1fr)) for 6 columns", () => {
       expect(css).toContain(
-        "grid-template-columns: repeat(5, minmax(260px, 1fr))",
+        "grid-template-columns: repeat(6, minmax(260px, 1fr))",
       );
     });
   });