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/tui/src/lib.rs

@@ -144,7 +144,6 @@ pub async fn run_main(
         config_profile: cli.config_profile.clone(),
         codex_linux_sandbox_exe,
         base_instructions: None,
-        include_plan_tool: Some(true),
         include_apply_patch_tool: None,
         include_view_image_tool: None,
         show_raw_agent_reasoning: cli.oss.then_some(true),