docs: delete agent-cost; rename kb-search skill to brain (#14)

* 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.

bin/tools/test_published_docs.py

@@ -43,7 +43,7 @@ class PublishedDocsTest(unittest.TestCase):
         paths = [
             ROOT / "README.md",
             ROOT / "docs" / "design.md",
-            ROOT / "skills" / "kb-search" / "SKILL.md",
+            ROOT / "skills" / "brain" / "SKILL.md",
             ROOT / "skills" / "diataxis-docs" / "SKILL.md",
         ]
         # Command-style `--hop 1` / `--hop N` plus follow/walk = the old lie.