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/tool_harness.rs

@@ -106,9 +106,7 @@ async fn update_plan_tool_emits_plan_update_event() -> anyhow::Result<()> {
 
     let server = start_mock_server().await;
 
-    let mut builder = test_codex().with_config(|config| {
-        config.features.enable(Feature::PlanTool);
-    });
+    let mut builder = test_codex();
     let TestCodex {
         codex,
         cwd,
@@ -193,9 +191,7 @@ async fn update_plan_tool_rejects_malformed_payload() -> anyhow::Result<()> {
 
     let server = start_mock_server().await;
 
-    let mut builder = test_codex().with_config(|config| {
-        config.features.enable(Feature::PlanTool);
-    });
+    let mut builder = test_codex();
     let TestCodex {
         codex,
         cwd,