feat: implement transcriber agent

Uses speech fragments of the VAD agent, emits transcribed text over SPADE's default communication channel to no recipient for now.

ref: N25B-209

src/control_backend/core/config.py

@@ -11,6 +11,7 @@ class AgentSettings(BaseModel):
     bdi_core_agent_name: str = "bdi_core"
     belief_collector_agent_name: str = "belief_collector"
     vad_agent_name: str = "vad_agent"
+    transcription_agent_name: str = "transcription_agent"
 
     ri_communication_agent_name: str = "ri_communication_agent"
     ri_command_agent_name: str = "ri_command_agent"