Rename host instruction payload to Instructions (#40709)

## What changed

Rename the `UserInstructions` type to `Instructions` across the extension API,
its core API re-export, and all consumers. This keeps loading behavior and the
`LoadedUserInstructions` provider interface unchanged.

GitOrigin-RevId: 1ac236b34c31718e7aa38ff5eae5e9b179aeedd8

codex-rs/core/src/session/mod.rs

@@ -1773,7 +1773,7 @@ impl Session {
             .clone()
     }
 
-    pub(crate) async fn user_instructions(&self) -> Option<codex_extension_api::UserInstructions> {
+    pub(crate) async fn user_instructions(&self) -> Option<codex_extension_api::Instructions> {
         self.services.agents_md_manager.user_instructions()
     }