Extract bot logic from MyMesh into MyMeshBot.h

Move tryBotReplyDM() and tryBotReplyChannel() implementations to
MyMeshBot.h (included at end of MyMesh.cpp). onMessageRecv and
onChannelMessageRecv now contain only a single-line call each,
minimising the diff against upstream in those methods.

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

examples/companion_radio/MyMesh.h

@@ -179,6 +179,9 @@ public:
 #endif
 
 private:
+  void tryBotReplyDM(const ContactInfo& from, const char* text);
+  void tryBotReplyChannel(uint8_t channel_idx, const char* text);
+
   void writeOKFrame();
   void writeErrFrame(uint8_t err_code);
   void writeDisabledFrame();