fix: System skills marker includes nested folders recursively (#10350)

Updated system skills bundled with Codex were not correctly replacing
the user's skills in their .system folder.

- Fix `.codex-system-skills.marker` not updating by hashing embedded
system skills recursively (nested dirs + file contents), so updates
trigger a reinstall.
- Added a build Cargo hook to rerun if there are changes in
`src/skills/assets/samples/*`, ensuring embedded skill updates rebuild
correctly under caching.
- Add a small unit test to ensure nested entries are included in the
fingerprint.

codex-rs/core/src/skills/system.rs

@@ -86,21 +86,8 @@ fn read_marker(path: &AbsolutePathBuf) -> Result<String, SystemSkillsError> {
 }
 
 fn embedded_system_skills_fingerprint() -> String {
-    let mut items: Vec<(String, Option<u64>)> = SYSTEM_SKILLS_DIR
-        .entries()
-        .iter()
-        .map(|entry| match entry {
-            include_dir::DirEntry::Dir(dir) => (dir.path().to_string_lossy().to_string(), None),
-            include_dir::DirEntry::File(file) => {
-                let mut file_hasher = DefaultHasher::new();
-                file.contents().hash(&mut file_hasher);
-                (
-                    file.path().to_string_lossy().to_string(),
-                    Some(file_hasher.finish()),
-                )
-            }
-        })
-        .collect();
+    let mut items = Vec::new();
+    collect_fingerprint_items(&SYSTEM_SKILLS_DIR, &mut items);
     items.sort_unstable_by(|(a, _), (b, _)| a.cmp(b));
 
     let mut hasher = DefaultHasher::new();
@@ -112,6 +99,25 @@ fn embedded_system_skills_fingerprint() -> String {
     format!("{:x}", hasher.finish())
 }
 
+fn collect_fingerprint_items(dir: &Dir<'_>, items: &mut Vec<(String, Option<u64>)>) {
+    for entry in dir.entries() {
+        match entry {
+            include_dir::DirEntry::Dir(subdir) => {
+                items.push((subdir.path().to_string_lossy().to_string(), None));
+                collect_fingerprint_items(subdir, items);
+            }
+            include_dir::DirEntry::File(file) => {
+                let mut file_hasher = DefaultHasher::new();
+                file.contents().hash(&mut file_hasher);
+                items.push((
+                    file.path().to_string_lossy().to_string(),
+                    Some(file_hasher.finish()),
+                ));
+            }
+        }
+    }
+}
+
 /// Writes the embedded `include_dir::Dir` to disk under `dest`.
 ///
 /// Preserves the embedded directory structure.
@@ -163,3 +169,28 @@ impl SystemSkillsError {
         Self::Io { action, source }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::SYSTEM_SKILLS_DIR;
+    use super::collect_fingerprint_items;
+
+    #[test]
+    fn fingerprint_traverses_nested_entries() {
+        let mut items = Vec::new();
+        collect_fingerprint_items(&SYSTEM_SKILLS_DIR, &mut items);
+        let mut paths: Vec<String> = items.into_iter().map(|(path, _)| path).collect();
+        paths.sort_unstable();
+
+        assert!(
+            paths
+                .binary_search_by(|probe| probe.as_str().cmp("skill-creator/SKILL.md"))
+                .is_ok()
+        );
+        assert!(
+            paths
+                .binary_search_by(|probe| probe.as_str().cmp("skill-creator/scripts/init_skill.py"))
+                .is_ok()
+        );
+    }
+}