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/bottom_pane/mod.rs

@@ -131,10 +131,19 @@ impl BottomPane {
         }
     }
 
+    pub fn set_skills(&mut self, skills: Option<Vec<SkillMetadata>>) {
+        self.composer.set_skill_mentions(skills);
+        self.request_redraw();
+    }
+
     pub fn status_widget(&self) -> Option<&StatusIndicatorWidget> {
         self.status.as_ref()
     }
 
+    pub fn skills(&self) -> Option<&Vec<SkillMetadata>> {
+        self.composer.skills()
+    }
+
     #[cfg(test)]
     pub(crate) fn context_window_percent(&self) -> Option<i64> {
         self.context_window_percent