feat: US-009 - Add state save atomicity with SetLastChannel

- Create pkg/state package with State and Manager structs
- Implement SetLastChannel with atomic save using temp file + rename
- Implement SetLastChatID with same atomic save pattern
- Add GetLastChannel, GetLastChatID, and GetTimestamp getters
- Use sync.RWMutex for thread-safe concurrent access
- Add comprehensive tests for atomic save, concurrent access, and persistence
- Cleanup temp file if rename fails

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

.ralph/prd.json

@@ -137,7 +137,7 @@
         "go test ./pkg/state -run TestAtomicSave passes"
       ],
       "priority": 9,
-      "passes": false,
+      "passes": true,
       "notes": ""
     },
     {