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_skills_bin_paths.py

@@ -0,0 +1,33 @@
+"""Every bin/ path named in skills/ must exist on disk."""
+from __future__ import annotations
+
+import re
+import unittest
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parents[2]
+BIN_PATH = re.compile(r"\b(bin/[A-Za-z0-9_./-]+)")
+
+
+class SkillsBinPathsTest(unittest.TestCase):
+    def test_agent_cost_skill_is_gone(self) -> None:
+        self.assertFalse(
+            (ROOT / "skills" / "agent-cost").exists(),
+            "skills/agent-cost documents bin/agents/cost which does not exist",
+        )
+
+    def test_brain_skill_replaces_kb_search(self) -> None:
+        self.assertTrue((ROOT / "skills" / "brain" / "SKILL.md").is_file())
+        self.assertFalse((ROOT / "skills" / "kb-search").exists())
+
+    def test_skill_bin_paths_exist(self) -> None:
+        missing: list[str] = []
+        for skill in sorted((ROOT / "skills").rglob("SKILL.md")):
+            text = skill.read_text()
+            for match in BIN_PATH.findall(text):
+                rel = match.rstrip("`'.,")
+                if rel.endswith(".go") or Path(rel).suffix == "" or Path(rel).suffix in {".go", ".py"}:
+                    p = ROOT / rel
+                    if not p.exists():
+                        missing.append(f"{skill.relative_to(ROOT)}: {rel}")
+        self.assertEqual(missing, [], "SKILL.md names bin/ paths that do not exist")