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/exec/tests/event_processor_with_json_output.rs

@@ -85,6 +85,7 @@ fn session_configured_produces_thread_started_event() {
             history_log_id: 0,
             history_entry_count: 0,
             initial_messages: None,
+            skill_load_outcome: None,
             rollout_path,
         }),
     );