Inject SKILL.md when it's explicitly mentioned. (#7763)

1. Skills load once in core at session start; the cached outcome is
reused across core and surfaced to TUI via SessionConfigured.
2. TUI detects explicit skill selections, and core injects the matching
SKILL.md content into the turn when a selected skill is present.

codex-rs/tui/src/app_backtrack.rs

@@ -350,7 +350,6 @@ impl App {
             auth_manager: self.auth_manager.clone(),
             models_manager: self.server.get_models_manager(),
             feedback: self.feedback.clone(),
-            skills: self.skills.clone(),
             is_first_run: false,
         };
         self.chat_widget =