Keep shared skill caches fresh across plugin loads (#37000)

## What changed

- Key cached skill snapshots by filesystem and plugin snapshot identity so compatible config and working-directory loads can share results without reusing stale plugin data.
- Coalesce concurrent loads for the same cache key and replace the cached entry on forced reload.
- Clear both plugin and skill caches when `skills/list` forces a reload, and bypass working-directory caching when effective plugin roots are present.

## Testing

- Cover concurrent cache sharing, filesystem isolation, forced reloads, and refreshed plugin skill metadata in host service and app server tests.

GitOrigin-RevId: 03fed3b40d45bb29206d5a3c3e78f06df04dbb1e

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

@@ -1,6 +1,8 @@
 use std::collections::HashMap;
 use std::collections::HashSet;
 use std::fmt;
+use std::hash::Hash;
+use std::hash::Hasher;
 use std::sync::Arc;
 use std::sync::Mutex;
 
@@ -33,6 +35,20 @@ impl PluginSkillSnapshots {
     }
 }
 
+impl PartialEq for PluginSkillSnapshots {
+    fn eq(&self, other: &Self) -> bool {
+        Arc::ptr_eq(&self.snapshots_by_root, &other.snapshots_by_root)
+    }
+}
+
+impl Eq for PluginSkillSnapshots {}
+
+impl Hash for PluginSkillSnapshots {
+    fn hash<H: Hasher>(&self, state: &mut H) {
+        Arc::as_ptr(&self.snapshots_by_root).hash(state);
+    }
+}
+
 impl fmt::Debug for PluginSkillSnapshots {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         f.debug_struct("PluginSkillSnapshots")