feat(grpc): dev mode, agent prefix, system prompt, error UX

- gRPC dev_mode config: disables JWT auth, uses fixed dev identity
- Agent prefix (agents.agent_prefix): dev agents use "dev-sol-orchestrator"
  to avoid colliding with production on shared Mistral accounts
- Coding sessions use instructions (system prompt + coding addendum)
  with mistral-medium-latest for personality adherence
- Conversations API: don't send both model + agent_id (422 fix)
- GrpcState carries system_prompt + orchestrator_agent_id
- Session.end() keeps session active for reuse (not "ended")
- User messages posted as m.notice, assistant as m.text (role detection)
- History loaded from Matrix room on session resume
- Docker Compose local dev stack: OpenSearch 3 + Tuwunel + SearXNG
- Dev config: localhost URLs, dev_mode, opensearch-init.sh for ML setup

src/conversations.rs

@@ -67,6 +67,11 @@ impl ConversationRegistry {
         *id = Some(agent_id);
     }
 
+    /// Get the current orchestrator agent ID, if set.
+    pub async fn get_agent_id(&self) -> Option<String> {
+        self.agent_id.lock().await.clone()
+    }
+
     /// Get or create a conversation for a room. Returns the conversation ID.
     /// If a conversation doesn't exist yet, creates one with the first message.
     /// `context_hint` is prepended to the first message on new conversations,