Read path is cgo like search. Control questions live in rank so CI can
test them without ladybug. Python bin/kb/{get,stats,eval} stays the
runner fallback.
* docs: delete agent-cost; rename kb-search skill to brain.
bin/agents/cost does not exist. CI unittest now fails if a SKILL.md names a
missing bin/ path.
* test: gate SKILL.md bin/ paths; name the brain skill brain.
Follow-up to the agent-cost delete: unittest fails if a skill names a missing
tool. Frontmatter name is brain, not kb-search.