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/dashboard/src/routes.test.ts

@@ -1987,7 +1987,7 @@ describe("Pause/Unpause endpoints", () => {
             id: "1234567890-abc123",
             text: "Please handle the edge case",
             createdAt: "2026-01-01T00:00:00.000Z",
-            author: "user" as const,
+            author: "user",
           },
         ],
       };