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/core/src/state/service.rs

@@ -4,6 +4,7 @@ use crate::AuthManager;
 use crate::RolloutRecorder;
 use crate::mcp_connection_manager::McpConnectionManager;
 use crate::openai_models::models_manager::ModelsManager;
+use crate::skills::SkillLoadOutcome;
 use crate::tools::sandboxing::ApprovalStore;
 use crate::unified_exec::UnifiedExecSessionManager;
 use crate::user_notification::UserNotifier;
@@ -24,4 +25,5 @@ pub(crate) struct SessionServices {
     pub(crate) models_manager: Arc<ModelsManager>,
     pub(crate) otel_event_manager: OtelEventManager,
     pub(crate) tool_approvals: Mutex<ApprovalStore>,
+    pub(crate) skills: Option<SkillLoadOutcome>,
 }