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/features/legacy.rs

@@ -29,10 +29,6 @@ const ALIASES: &[Alias] = &[
         legacy_key: "include_apply_patch_tool",
         feature: Feature::ApplyPatchFreeform,
     },
-    Alias {
-        legacy_key: "include_plan_tool",
-        feature: Feature::PlanTool,
-    },
     Alias {
         legacy_key: "include_view_image_tool",
         feature: Feature::ViewImageTool,
@@ -55,7 +51,6 @@ pub(crate) fn feature_for_key(key: &str) -> Option<Feature> {
 
 #[derive(Debug, Default)]
 pub struct LegacyFeatureToggles {
-    pub include_plan_tool: Option<bool>,
     pub include_apply_patch_tool: Option<bool>,
     pub include_view_image_tool: Option<bool>,
     pub experimental_use_freeform_apply_patch: Option<bool>,
@@ -68,12 +63,6 @@ pub struct LegacyFeatureToggles {
 
 impl LegacyFeatureToggles {
     pub fn apply(self, features: &mut Features) {
-        set_if_some(
-            features,
-            Feature::PlanTool,
-            self.include_plan_tool,
-            "include_plan_tool",
-        );
         set_if_some(
             features,
             Feature::ApplyPatchFreeform,