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/tests/suite/model_tools.rs

@@ -57,7 +57,6 @@ async fn collect_tool_identifiers_for_model(model: &str) -> Vec<String> {
     config.model = model.to_string();
     config.model_family =
         find_family_for_model(model).unwrap_or_else(|| panic!("unknown model family for {model}"));
-    config.features.disable(Feature::PlanTool);
     config.features.disable(Feature::ApplyPatchFreeform);
     config.features.disable(Feature::ViewImageTool);
     config.features.disable(Feature::WebSearchRequest);
@@ -98,7 +97,8 @@ async fn model_selects_expected_tools() {
             "local_shell".to_string(),
             "list_mcp_resources".to_string(),
             "list_mcp_resource_templates".to_string(),
-            "read_mcp_resource".to_string()
+            "read_mcp_resource".to_string(),
+            "update_plan".to_string()
         ],
         "codex-mini-latest should expose the local shell tool",
     );
@@ -110,7 +110,8 @@ async fn model_selects_expected_tools() {
             "shell".to_string(),
             "list_mcp_resources".to_string(),
             "list_mcp_resource_templates".to_string(),
-            "read_mcp_resource".to_string()
+            "read_mcp_resource".to_string(),
+            "update_plan".to_string()
         ],
         "o3 should expose the generic shell tool",
     );
@@ -123,6 +124,7 @@ async fn model_selects_expected_tools() {
             "list_mcp_resources".to_string(),
             "list_mcp_resource_templates".to_string(),
             "read_mcp_resource".to_string(),
+            "update_plan".to_string(),
             "apply_patch".to_string()
         ],
         "gpt-5-codex should expose the apply_patch tool",