feat: US-010 - Add RecordLastChannel to AgentLoop with atomic state save

- Add state *state.Manager field to AgentLoop struct
- Initialize stateManager in NewAgentLoop using state.NewManager
- Implement RecordLastChannel method that calls state.SetLastChannel
- Implement RecordLastChatID method for chat ID tracking
- Add comprehensive tests for state persistence
- Verify state survives across AgentLoop instances

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.ralph/prd.json

@@ -151,7 +151,7 @@
         "go test ./pkg/agent -run TestRecordLastChannel passes"
       ],
       "priority": 10,
-      "passes": false,
+      "passes": true,
       "notes": ""
     },
     {