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/tests/suite/mod.rs

@@ -50,6 +50,7 @@ mod seatbelt;
 mod shell_command;
 mod shell_serialization;
 mod shell_snapshot;
+mod skills;
 mod stream_error_allows_next_turn;
 mod stream_no_completed;
 mod text_encoding_fix;