Extract codex-core-skills crate (#15749)

## Summary
- move skill loading and management into codex-core-skills
- leave codex-core with the thin integration layer and shared wiring

## Testing
- CI

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/plugin/src/plugin_namespace.rs

@@ -0,0 +1,70 @@
+//! Resolve plugin namespace from skill file paths by walking ancestors for `plugin.json`.
+
+use std::fs;
+use std::path::Path;
+
+/// Relative path from a plugin root to its manifest file.
+pub const PLUGIN_MANIFEST_PATH: &str = ".codex-plugin/plugin.json";
+
+#[derive(serde::Deserialize)]
+#[serde(rename_all = "camelCase")]
+struct RawPluginManifestName {
+    #[serde(default)]
+    name: String,
+}
+
+fn plugin_manifest_name(plugin_root: &Path) -> Option<String> {
+    let manifest_path = plugin_root.join(PLUGIN_MANIFEST_PATH);
+    if !manifest_path.is_file() {
+        return None;
+    }
+    let contents = fs::read_to_string(&manifest_path).ok()?;
+    let RawPluginManifestName { name: raw_name } = serde_json::from_str(&contents).ok()?;
+    Some(
+        plugin_root
+            .file_name()
+            .and_then(|entry| entry.to_str())
+            .filter(|_| raw_name.trim().is_empty())
+            .unwrap_or(raw_name.as_str())
+            .to_string(),
+    )
+}
+
+/// Returns the plugin manifest `name` for the nearest ancestor of `path` that contains a valid
+/// plugin manifest (same `name` rules as full manifest loading in codex-core).
+pub fn plugin_namespace_for_skill_path(path: &Path) -> Option<String> {
+    for ancestor in path.ancestors() {
+        if let Some(name) = plugin_manifest_name(ancestor) {
+            return Some(name);
+        }
+    }
+    None
+}
+
+#[cfg(test)]
+mod tests {
+    use super::plugin_namespace_for_skill_path;
+    use std::fs;
+    use tempfile::tempdir;
+
+    #[test]
+    fn uses_manifest_name() {
+        let tmp = tempdir().expect("tempdir");
+        let plugin_root = tmp.path().join("plugins/sample");
+        let skill_path = plugin_root.join("skills/search/SKILL.md");
+
+        fs::create_dir_all(skill_path.parent().expect("parent")).expect("mkdir");
+        fs::create_dir_all(plugin_root.join(".codex-plugin")).expect("mkdir manifest");
+        fs::write(
+            plugin_root.join(".codex-plugin/plugin.json"),
+            r#"{"name":"sample"}"#,
+        )
+        .expect("write manifest");
+        fs::write(&skill_path, "---\ndescription: search\n---\n").expect("write skill");
+
+        assert_eq!(
+            plugin_namespace_for_skill_path(&skill_path),
+            Some("sample".to_string())
+        );
+    }
+}