refactor(HAI-093): extract AgentLogger abstraction and rewire agents

- Add AgentLogger class encapsulating log file management and structured logging
- Add comprehensive unit tests for AgentLogger
- Rewire executor, triage, and merger to use AgentLogger instead of inline logging
- Reduce duplication across agent modules
- Export AgentLogger from engine package index

packages/engine/src/merger.test.ts

@@ -242,7 +242,7 @@ describe("aiMergeTask — agent log persistence", () => {
 
     await aiMergeTask(store, "/tmp/root", "HAI-050");
 
-    expect(store.appendAgentLog).toHaveBeenCalledWith("HAI-050", "Bash", "tool");
+    expect(store.appendAgentLog).toHaveBeenCalledWith("HAI-050", "Bash", "tool", "git status");
   });
 
   it("still fires onAgentText callback alongside logging", async () => {