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/mcp-server/src/outgoing_message.rs

@@ -266,6 +266,7 @@ mod tests {
                 history_log_id: 1,
                 history_entry_count: 1000,
                 initial_messages: None,
+                skill_load_outcome: None,
                 rollout_path: rollout_file.path().to_path_buf(),
             }),
         };
@@ -305,6 +306,7 @@ mod tests {
             history_log_id: 1,
             history_entry_count: 1000,
             initial_messages: None,
+            skill_load_outcome: None,
             rollout_path: rollout_file.path().to_path_buf(),
         };
         let event = Event {