feat(KB-341): unify steeringComments and comments into single comments system

- Consolidate steeringComments and comments into unified comments field using TaskComment type

- Remove SteeringComment type, migrate all comments to TaskComment format

- Update addComment to delegate to addTaskComment with auto-refinement for done tasks

- Merge legacy steeringComments data into comments during database migration

- Update dashboard API endpoint from /steer to /comments

- Update executor to use unified comments field for real-time injection

- Update PR comment handler to use addTaskComment method

packages/core/src/db-migrate.ts

@@ -224,7 +224,10 @@ async function migrateTasks(kbDir: string, db: Database): Promise<void> {
         toJson(task.steps || []),
         toJson(task.log || []),
         toJson(task.attachments || []),
-        toJson(mergedComments),
+        toJson(task.attachments || []),
+        "[]", // steeringComments column - no longer used, write empty array
+        // Merge legacy steeringComments into unified comments field during migration
+        toJson([...((task as any).steeringComments || []), ...(task.comments || [])]),
         toJson(task.workflowStepResults || []),
         toJsonNullable(task.prInfo),
         toJsonNullable(task.issueInfo),