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/exec/src/cli.rs

@@ -67,10 +67,6 @@ pub struct Cli {
     #[arg(long = "json", alias = "experimental-json", default_value_t = false)]
     pub json: bool,
 
-    /// Whether to include the plan tool in the conversation.
-    #[arg(long = "include-plan-tool")]
-    pub include_plan_tool: Option<bool>,
-
     /// Specifies file where the last message from the agent should be written.
     #[arg(long = "output-last-message", short = 'o', value_name = "FILE")]
     pub last_message_file: Option<PathBuf>,

codex-rs/exec/src/lib.rs

@@ -70,14 +70,9 @@ pub async fn run_main(cli: Cli, codex_linux_sandbox_exe: Option<PathBuf>) -> any
         sandbox_mode: sandbox_mode_cli_arg,
         prompt,
         output_schema: output_schema_path,
-        include_plan_tool,
         config_overrides,
     } = cli;
 
-    if include_plan_tool.is_some() {
-        eprintln!("include-plan-tool is deprecated. Plan tool is now enabled by default.");
-    }
-
     // Determine the prompt source (parent or subcommand) and read from stdin if needed.
     let prompt_arg = match &command {
         // Allow prompt before the subcommand by falling back to the parent-level prompt
@@ -181,7 +176,6 @@ pub async fn run_main(cli: Cli, codex_linux_sandbox_exe: Option<PathBuf>) -> any
         model_provider,
         codex_linux_sandbox_exe,
         base_instructions: None,
-        include_plan_tool: Some(include_plan_tool.unwrap_or(true)),
         include_apply_patch_tool: None,
         include_view_image_tool: None,
         show_raw_agent_reasoning: oss.then_some(true),