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/chat_composer.rs

@@ -801,6 +801,10 @@ impl ChatComposer {
         self.skills.as_ref().is_some_and(|s| !s.is_empty())
     }
 
+    pub fn skills(&self) -> Option<&Vec<SkillMetadata>> {
+        self.skills.as_ref()
+    }
+
     /// Extract a token prefixed with `prefix` under the cursor, if any.
     ///
     /// The returned string **does not** include the prefix.