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.

outputs/x86_64-linux/tests/agent-profile/expected.nix

@@ -2,6 +2,7 @@
 let
   common = {
     agentsFromHome = true;
+    qwenInstalled = true;
     devToolsFromHome = true;
     nushellSecrets = false;
     nixLd = true;

outputs/x86_64-linux/tests/agent-profile/expr.nix

@@ -23,6 +23,7 @@ lib.genAttrs hosts (
   {
     # home role: home/linux/tui.nix
     agentsFromHome = hasPackage "codex" home.home.packages;
+    qwenInstalled = hasPackage "qwen-code" home.home.packages;
     devToolsFromHome = hasPackage "kubectl" home.home.packages;
     # ~/.secrets is synced to the desktops only; sourcing it would break nushell.
     nushellSecrets = home.modules.shell.secrets.enable;

outputs/x86_64-linux/tests/idols-ai-gpu/expected.nix

@@ -19,4 +19,5 @@
   wivrnImportsOpenXRToSteam = true;
   wivrnStatePreserved = true;
   adbStatePreserved = true;
+  qwenStatePreserved = true;
 }

outputs/x86_64-linux/tests/idols-ai-gpu/expr.nix

@@ -54,4 +54,5 @@ in
     cfg.environment.sessionVariables.PRESSURE_VESSEL_IMPORT_OPENXR_1_RUNTIMES == "1";
   wivrnStatePreserved = privatelyPreserved ".config/wivrn";
   adbStatePreserved = privatelyPreserved ".android";
+  qwenStatePreserved = privatelyPreserved ".qwen";
 }

outputs/x86_64-linux/tests/idols-aquamarine/expr.nix

@@ -102,6 +102,7 @@ in
       ".local/state/home-manager"
       ".local/state/nix/profiles"
       ".codex"
+      ".qwen"
       ".config/opencode"
     ];
   pythonInstalled = builtins.any (