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/app-server/src/codex_message_processor.rs

@@ -1421,7 +1421,6 @@ async fn derive_config_from_params(
         sandbox: sandbox_mode,
         config: cli_overrides,
         base_instructions,
-        include_plan_tool,
         include_apply_patch_tool,
     } = params;
     let overrides = ConfigOverrides {
@@ -1434,7 +1433,6 @@ async fn derive_config_from_params(
         model_provider: None,
         codex_linux_sandbox_exe,
         base_instructions,
-        include_plan_tool,
         include_apply_patch_tool,
         include_view_image_tool: None,
         show_raw_agent_reasoning: None,