feat: US-007 - Add heartbeat async task execution support

- Add local ToolResult struct definition to avoid circular dependencies
- Define HeartbeatHandler function type for tool-supporting callbacks
- Add SetOnHeartbeatWithTools method to configure new handler
- Add ExecuteHeartbeatWithTools public method
- Add internal executeHeartbeatWithTools implementation
- Update checkHeartbeat to prefer new tool-supporting handler
- Detect and handle async tasks (log and return immediately)
- Handle error results with proper logging
- Add comprehensive tests for async, error, sync, and nil result cases

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

.ralph/prd.json

@@ -107,7 +107,7 @@
         "go test ./pkg/heartbeat -run TestAsync passes"
       ],
       "priority": 7,
-      "passes": false,
+      "passes": true,
       "notes": ""
     },
     {