fix(herdr): guard idle-pane env wrapping on explicit env_flags in new-session

- Prevent injecting 'env HERDR_WORKSPACE_ID=...' into final_cmd when caller passed no environment variables
- Avoid argument parse errors in real agent binaries (e.g. grok, agy) when reusing an idle pane in new-session

.agents/skills/lib.sh

@@ -740,7 +740,7 @@ except Exception:
 
       if [ -n "$idle_pane" ]; then
         target_pane="$idle_pane"
-        if [ -n "$final_cmd" ]; then
+        if [ -n "$env_flags" ] && [ -n "$final_cmd" ]; then
           idle_env="${env_flags//--env /}"
           idle_env="${idle_env:+$idle_env }HERDR_WORKSPACE_ID=$existing_ws"
           final_cmd="env $idle_env $final_cmd"