Support SYSTEM skills. (#8220)

1. Remove PUBLIC skills and introduce SYSTEM skills embedded in the
binary and installed into $CODEX_HOME/skills/.system at startup.
2. Skills are now always enabled (feature flag removed).
3. Update skills/list to accept forceReload and plumb it through (not
used by clients yet).

codex-rs/protocol/src/protocol.rs

@@ -1690,7 +1690,7 @@ pub struct ListSkillsResponseEvent {
 pub enum SkillScope {
     User,
     Repo,
-    Public,
+    System,
 }
 
 #[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS)]