chore(multiagent) skills instructions toggle (#18596)

## Summary
Support toggling the skills message off.

## Test Plan
- [x] Updated unit tests

codex-rs/config/src/skills_config.rs

@@ -27,6 +27,10 @@ pub struct SkillsConfig {
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub bundled: Option<BundledSkillsConfig>,
 
+    /// Whether turns receive the automatic skills instructions block.
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub include_instructions: Option<bool>,
+
     #[serde(default, skip_serializing_if = "Vec::is_empty")]
     pub config: Vec<SkillConfig>,
 }