Load host skill roots through the skills extension (#37162)

## What changed

- Load ordinary recursive host skill roots with the skills extension's host loader.
- Keep plugin-specific roots on the existing loader so plugin snapshot caching and namespacing continue to work.
- Merge both kinds of root snapshots through the existing precedence and deduplication logic, including discovery paths and source filesystem mappings.
- Move `HostSkillsSnapshot` into the skills extension as the owner of host skill loading.

## Testing

- Cover merged host and plugin catalogs, precedence for symlinked roots, discovery paths, and end-to-end rendering and invocation of host, repository, and plugin skills.

GitOrigin-RevId: c18080afb917c33c57bc17e36b40dd0f3c72d1d7

codex-rs/ext/skills/src/host_snapshot.rs

@@ -0,0 +1,25 @@
+use std::io;
+use std::sync::Arc;
+
+use codex_core_skills::SkillLoadOutcome;
+use codex_skills::SkillMetadata;
+
+/// Immutable snapshot of host-owned skills and their source filesystems.
+#[derive(Debug, Clone)]
+pub struct HostSkillsSnapshot {
+    outcome: Arc<SkillLoadOutcome>,
+}
+
+impl HostSkillsSnapshot {
+    pub fn new(outcome: Arc<SkillLoadOutcome>) -> Self {
+        Self { outcome }
+    }
+
+    pub fn outcome(&self) -> &SkillLoadOutcome {
+        self.outcome.as_ref()
+    }
+
+    pub async fn read_skill_text(&self, skill: &SkillMetadata) -> io::Result<String> {
+        self.outcome.read_skill_text(skill).await
+    }
+}