Load plugin skill roots through the host skills service (#37440)

## What changed

- Implement `SkillRootLoader<PluginSkillRoot>` for `HostSkillsService`.
- Load roots concurrently while preserving request order, product restrictions, cached snapshots, skill errors, and per-skill filesystem metadata.
- Deduplicate overlapping roots and prefer native plugin skills over migrated commands with the same plugin ID and skill name.

## Testing

- Cover native and migrated skill precedence, nested and overlapping roots, product filtering, snapshot reuse, error propagation, and symlinked migration roots.

GitOrigin-RevId: 2270d10616b3a0463852c563ee071c524cd11e6b

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

@@ -1,6 +1,5 @@
 use crate::app_mcp_routing::apply_app_mcp_routing_policy;
 use crate::app_mcp_routing::apps_route_available;
-use crate::command_migration::migrated_command_skills_root;
 use crate::is_openai_curated_marketplace_name;
 use crate::manifest::PluginManifest;
 use crate::manifest::PluginManifestFormat;
@@ -53,6 +52,7 @@ use codex_utils_absolute_path::AbsolutePathBuf;
 use codex_utils_plugins::PluginIdentity;
 use codex_utils_plugins::SkillDiscoveryMode;
 use codex_utils_plugins::find_plugin_manifest_path;
+use codex_utils_plugins::migrated_command_skills_root;
 use serde_json::Value as JsonValue;
 use std::collections::HashMap;
 use std::collections::HashSet;