Expose executor skill roots from skills.read (#38268)

## Why

Executor-backed skills can include bundled scripts, so skill readers need the
skill directory in the executor filesystem to locate them.

## What changed

- Add `skill_root` to `skills.read` responses for executor-backed skills.
- Derive the root from the parent directory of the skill's main resource and
  omit the field for other skill authorities.

## Testing

- Verify executor skill reads return the expected root for both `SKILL.md` and
  a referenced resource.

GitOrigin-RevId: b7035c7e2e62155890795fb508fefe3a1c17805b

codex-rs/ext/skills/src/tools/read.rs

@@ -40,6 +40,8 @@ struct ReadArgs {
 struct ReadResponse {
     resource: String,
     contents: String,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    skill_root: Option<String>,
     next_cursor: Option<String>,
 }
 
@@ -56,7 +58,7 @@ impl ToolExecutor<ToolCall> for ReadTool {
     fn spec(&self) -> ToolSpec {
         skill_function_tool::<ReadArgs, ReadResponse>(
             TOOL_NAME,
-            "Read one page from a skill. Pass its provided package directly; root aliases are resolved automatically. Omit resource to read SKILL.md; to read another file, use the same package and pass the file's complete skill:// identifier as resource. If the package is not provided, use skills.list to find it. Pass next_cursor back as cursor to continue.",
+            "Read one page from a skill. Pass its provided package directly; root aliases are resolved automatically. Omit resource to read SKILL.md; to read another file, use the same package and pass the file's complete skill:// identifier as resource. For executor-backed skills, skill_root is the skill's absolute directory in the executor filesystem and can be used to locate bundled scripts. If the package is not provided, use skills.list to find it. Pass next_cursor back as cursor to continue.",
         )
     }
 
@@ -189,7 +191,20 @@ impl ToolExecutor<ToolCall> for ReadTool {
                     "skills.read cursor is invalid".to_string(),
                 ));
             }
-            let response = page_response(result.resource.as_str(), &result.contents, start)?;
+            let skill_root = if output_authority == super::SkillToolAuthoritySelector::Executor {
+                main_prompt
+                    .environment_path()
+                    .and_then(|(_, path)| path.parent())
+                    .map(|path| path.inferred_native_path_string())
+            } else {
+                None
+            };
+            let response = page_response(
+                result.resource.as_str(),
+                &result.contents,
+                skill_root.as_deref(),
+                start,
+            )?;
             let output = skill_json_output(&response, output_authority)?;
 
             if requested_resource == main_prompt
@@ -212,11 +227,13 @@ impl ToolExecutor<ToolCall> for ReadTool {
 fn page_response(
     resource: &str,
     contents: &str,
+    skill_root: Option<&str>,
     start: usize,
 ) -> Result<ReadResponse, FunctionCallError> {
     let response = |end, next_cursor| ReadResponse {
         resource: resource.to_string(),
         contents: contents[start..end].to_string(),
+        skill_root: skill_root.map(str::to_string),
         next_cursor,
     };
     let complete = response(contents.len(), None);