feat: list_dir tool (#4817)

Add a tool to list_dir. It is useful because we can mark it as
non-mutating and so use it in parallel

Author: jif-oai

codex-rs/core/src/model_family.rs

@@ -120,6 +120,7 @@ pub fn find_family_for_model(mut slug: &str) -> Option<ModelFamily> {
             base_instructions: GPT_5_CODEX_INSTRUCTIONS.to_string(),
             experimental_supported_tools: vec![
                 "read_file".to_string(),
+                "list_dir".to_string(),
                 "test_sync_tool".to_string()
             ],
             supports_parallel_tool_calls: true,
@@ -133,7 +134,7 @@ pub fn find_family_for_model(mut slug: &str) -> Option<ModelFamily> {
             reasoning_summary_format: ReasoningSummaryFormat::Experimental,
             base_instructions: GPT_5_CODEX_INSTRUCTIONS.to_string(),
             apply_patch_tool_type: Some(ApplyPatchToolType::Freeform),
-            experimental_supported_tools: vec!["read_file".to_string()],
+            experimental_supported_tools: vec!["read_file".to_string(), "list_dir".to_string()],
             supports_parallel_tool_calls: true,
         )