Enable plan tool by default (#5384)

## Summary
- make the plan tool available by default by removing the feature flag
and always registering the handler
- drop plan-tool CLI and API toggles across the exec, TUI, MCP server,
and app server code paths
- update tests and configs to reflect the always-on plan tool and guard
workspace restriction tests against env leakage

## Testing
Manually tested the extension. 
------
https://chatgpt.com/codex/tasks/task_i_68f67a3ff2d083209562a773f814c1f9

codex-rs/core/src/codex.rs

@@ -1386,9 +1386,8 @@ async fn spawn_review_thread(
     let model = config.review_model.clone();
     let review_model_family = find_family_for_model(&model)
         .unwrap_or_else(|| parent_turn_context.client.get_model_family());
-    // For reviews, disable plan, web_search, view_image regardless of global settings.
+    // For reviews, disable web_search and view_image regardless of global settings.
     let mut review_features = config.features.clone();
-    review_features.disable(crate::features::Feature::PlanTool);
     review_features.disable(crate::features::Feature::WebSearchRequest);
     review_features.disable(crate::features::Feature::ViewImageTool);
     review_features.disable(crate::features::Feature::StreamableShell);