Handle invalid plugin skills manifest field (#25717)

## Summary
- Treat invalid `plugin.json` `skills` shapes as a field-level warning
instead of rejecting the whole manifest
- Keep valid string path behavior unchanged and continue falling back to
the default `skills/` root
- Add regression coverage for array-shaped `skills`

## Tests
- `just fmt`
- `cargo test -p codex-core-plugins`

codex-rs/core-plugins/src/manager_tests.rs

@@ -959,6 +959,43 @@ async fn load_plugins_ignores_manifest_component_paths_without_dot_slash() {
     );
 }
 
+#[tokio::test]
+async fn load_plugins_ignores_invalid_manifest_skills_shape() {
+    let codex_home = TempDir::new().unwrap();
+    let plugin_root = codex_home
+        .path()
+        .join("plugins/cache")
+        .join("test/sample/local");
+
+    write_file(
+        &plugin_root.join(".codex-plugin/plugin.json"),
+        r#"{
+  "name": "sample",
+  "skills": ["./custom-skills/"]
+}"#,
+    );
+    write_file(
+        &plugin_root.join("skills/default-skill/SKILL.md"),
+        "---\nname: default-skill\ndescription: default skill\n---\n",
+    );
+    write_file(
+        &plugin_root.join("custom-skills/custom-skill/SKILL.md"),
+        "---\nname: custom-skill\ndescription: custom skill\n---\n",
+    );
+
+    let outcome = load_plugins_from_config(
+        &plugin_config_toml(/*enabled*/ true, /*plugins_feature_enabled*/ true),
+        codex_home.path(),
+    )
+    .await;
+
+    assert_eq!(outcome.plugins()[0].error, None);
+    assert_eq!(
+        outcome.plugins()[0].skill_roots,
+        vec![plugin_root.join("skills").abs()]
+    );
+}
+
 #[tokio::test]
 async fn load_plugins_preserves_disabled_plugins_without_effective_contributions() {
     let codex_home = TempDir::new().unwrap();

codex-rs/core-plugins/src/manifest.rs

@@ -23,7 +23,7 @@ struct RawPluginManifest {
     // Keep manifest paths as raw strings so we can validate the required `./...` syntax before
     // resolving them under the plugin root.
     #[serde(default)]
-    skills: Option<String>,
+    skills: Option<RawPluginManifestPath>,
     #[serde(default)]
     mcp_servers: Option<String>,
     #[serde(default)]
@@ -127,6 +127,13 @@ enum RawPluginManifestDefaultPromptEntry {
     Invalid(JsonValue),
 }
 
+#[derive(Debug, Deserialize)]
+#[serde(untagged)]
+enum RawPluginManifestPath {
+    Path(String),
+    Invalid(JsonValue),
+}
+
 #[derive(Debug, Deserialize)]
 #[serde(untagged)]
 enum RawPluginManifestHooks {
@@ -238,7 +245,7 @@ pub fn load_plugin_manifest(plugin_root: &Path) -> Option<PluginManifest> {
                 description,
                 keywords,
                 paths: PluginManifestPaths {
-                    skills: resolve_manifest_path(plugin_root, "skills", skills.as_deref()),
+                    skills: resolve_manifest_path_value(plugin_root, "skills", skills.as_ref()),
                     mcp_servers: resolve_manifest_path(
                         plugin_root,
                         "mcpServers",
@@ -394,6 +401,23 @@ fn json_value_type(value: &JsonValue) -> &'static str {
     }
 }
 
+fn resolve_manifest_path_value(
+    plugin_root: &Path,
+    field: &'static str,
+    path: Option<&RawPluginManifestPath>,
+) -> Option<AbsolutePathBuf> {
+    match path? {
+        RawPluginManifestPath::Path(path) => resolve_manifest_path(plugin_root, field, Some(path)),
+        RawPluginManifestPath::Invalid(value) => {
+            tracing::warn!(
+                "ignoring {field}: expected a string; found {}",
+                json_value_type(value)
+            );
+            None
+        }
+    }
+}
+
 fn resolve_manifest_path(
     plugin_root: &Path,
     field: &'static str,