* 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.
Commands live at bin/brain/{index,get,stats,eval,watch}.go and
bin/mail/import.go, bin/markdown/import.go, bin/postgres/query.go.
Python remains the Ladybug write worker. index_mail is a deprecation
shim that rebuilds via --with-mail.
Published docs and skills still taught bin/kb/search --hop 1. Search lives
at bin/brain/search.go; --hop errors until File edges exist. A unittest
gates the SoT so the lie cannot return.