feat: US-011 - Add MessageTool tests

- Added comprehensive test suite for MessageTool (message_test.go)
- 10 test cases covering all acceptance criteria:
  - Success returns SilentResult with proper ForLLM status
  - ForUser is empty (user receives message directly)
  - Failure returns ErrorResult with IsError=true
  - Custom channel/chat_id parameter handling
  - Error scenarios (missing content, no target, not configured)

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

.ralph/prd.json

@@ -167,7 +167,7 @@
         "go test ./pkg/tools -run TestMessageTool passes"
       ],
       "priority": 11,
-      "passes": false,
+      "passes": true,
       "notes": ""
     },
     {