_handle_message() re-checks a slash-skill command's per-platform disabled
status before dispatch, because get_skill_commands() only applies the
global disabled list at scan time. That check only covered the leading
skill: split_stacked_skill_commands() resolves additional /skill tokens
that follow it (stacked invocations, up to 5 skills, #57987), and
build_stacked_skill_invocation_message() loads every one of them via
_load_skill_payload() with no disabled-status check of any kind.
A message on a platform with skills.platform_disabled configured for a
given skill could still get that skill's full SKILL.md content injected
into the agent's context for the turn, as long as it was typed after an
allowed skill: `/allowed-skill /disabled-skill do X`.
Fix: after computing the stacked extra_keys, look up each one's skill
name and re-check it against the same get_disabled_skill_names(platform=)
set already used for the leading skill. If any stacked skill is disabled
for the platform, reject the whole invocation with the same style of
message the leading-skill check already returns, instead of partially
loading it.
