feat(KB-622): unify steeringComments and comments into single comments field

- Merge steeringComments and comments into unified comments field in Task type
- Update TaskStore to use single comments array instead of separate steeringComments
- Add database migration to convert existing steeringComments to comments
- Update executor to inject all comments into AI execution context
- Update dashboard SteeringTab to use unified comments API
- Update CLI task steer command to use comments field
- Update PR comment handler to add comments via unified API

packages/core/src/db-migrate.test.ts

@@ -541,7 +541,8 @@ describe("migrateFromLegacy", () => {
       expect(JSON.parse(row.steps)).toHaveLength(2);
       expect(JSON.parse(row.log)).toHaveLength(1);
       expect(JSON.parse(row.attachments)).toHaveLength(1);
-      expect(JSON.parse(row.steeringComments)).toHaveLength(1);
+      // steeringComments should be merged into comments
+      expect(JSON.parse(row.comments)).toHaveLength(1); // 1 steering comment migrated
       expect(JSON.parse(row.workflowStepResults)).toHaveLength(1);
       expect(JSON.parse(row.prInfo).number).toBe(1);
       expect(JSON.parse(row.issueInfo).number).toBe(10);