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/protocol/src/user_input.rs

@@ -21,4 +21,10 @@ pub enum UserInput {
     LocalImage {
         path: std::path::PathBuf,
     },
+
+    /// Skill selected by the user (name + path to SKILL.md).
+    Skill {
+        name: String,
+        path: std::path::PathBuf,
+    },
 }