feat(agents): add Qwen Code and use OpenCode V2 as opencode

Expose llm-agents' opencode2 package under the normal opencode command and persist Qwen Code's ~/.qwen state on the ephemeral-root hosts.

hosts/idols-ai/preservation.nix

@@ -178,6 +178,12 @@ in
         ".codex"
         ".dsh" # DeepSeek Harness
         ".pi"
+        # Qwen Code keeps settings, credentials, history, sessions, and logs
+        # under ~/.qwen; keep the whole directory private and persistent.
+        {
+          directory = ".qwen";
+          mode = "0700";
+        }
         ".config/opencode"
         ".local/share/opencode"
         ".local/state/opencode"