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>,