feat(FN-1532): add performance indexes for dashboard boot queries

- Add composite indexes for dashboard boot query paths
- Optimize taskList queries with (column, updatedAt) and (column, status, updatedAt) indexes
- Optimize activityLog queries with (timestamp DESC, taskId) index
- Add idempotent CREATE INDEX IF NOT EXISTS pattern for migrations
- Document index patterns in memory.md for future schema changes
- Update schema version assertions in db.test.ts and task-documents.test.ts
- Add changeset for dashboard load performance improvement

packages/core/src/__tests__/task-documents.test.ts

@@ -51,7 +51,7 @@ describe("TaskStore task documents", () => {
 
     expect(tableNames.has("task_documents")).toBe(true);
     expect(tableNames.has("task_document_revisions")).toBe(true);
-    expect(db.getSchemaVersion()).toBe(27);
+    expect(db.getSchemaVersion()).toBe(28);
 
     const index = db
       .prepare(