A /skill invocation expands into a message that embeds the whole skill body.
Anything that summarizes a user turn from its raw content reads the skill's
prose as if the user had written it.
describe_skill_invocation() sits next to the existing extractor and reuses its
markers, returning `/work — fix the title leak` for an invocation with an
instruction and `/work` for a bare one. It also exports the SQL LIKE pattern
and excerpt-joint sentinel that listing queries need to recognize scaffolding
before a row reaches Python.
