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-protocol/src/protocol.rs

@@ -247,10 +247,6 @@ pub struct NewConversationParams {
     #[serde(skip_serializing_if = "Option::is_none")]
     pub base_instructions: Option<String>,
 
-    /// Whether to include the plan tool in the conversation.
-    #[serde(skip_serializing_if = "Option::is_none")]
-    pub include_plan_tool: Option<bool>,
-
     /// Whether to include the apply patch tool in the conversation.
     #[serde(skip_serializing_if = "Option::is_none")]
     pub include_apply_patch_tool: Option<bool>,
@@ -886,7 +882,6 @@ mod tests {
                 sandbox: None,
                 config: None,
                 base_instructions: None,
-                include_plan_tool: None,
                 include_apply_patch_tool: None,
             },
         };