refactor: one Go module; brain search in bin/brain + internal/brain. (#8)

Collapse nested kbsearch/chats go.mod into the root module. Ranking stays
cgo-free under internal/brain/rank so CI does not need ladybug. bin/kb/search
is a deprecation wrapper that still sets CGO and builds the binary.

bin/tools/test_bin_layout.py

@@ -0,0 +1,36 @@
+"""D14 layout: bin/{subject}/{method}.go, libs in internal/, one go.mod."""
+from __future__ import annotations
+
+import unittest
+from pathlib import Path
+
+ROOT = Path(__file__).resolve().parents[2]
+
+
+class BinLayoutTest(unittest.TestCase):
+    def test_brain_search_shebang_exists(self) -> None:
+        p = ROOT / "bin" / "brain" / "search.go"
+        self.assertTrue(p.is_file(), "missing bin/brain/search.go")
+        first = p.read_text().splitlines()[0]
+        self.assertTrue(
+            first.startswith("//usr/bin/env go run"),
+            f"shebang first line, got {first!r}",
+        )
+
+    def test_no_nested_go_mod_under_bin(self) -> None:
+        nested = list((ROOT / "bin").rglob("go.mod"))
+        self.assertEqual(nested, [], f"nested go.mod files: {nested}")
+
+    def test_rank_lives_in_internal_brain(self) -> None:
+        self.assertTrue(
+            (ROOT / "internal" / "brain" / "rank" / "rank.go").is_file(),
+            "ranking must live in internal/brain/rank (cgo-free)",
+        )
+        self.assertFalse(
+            (ROOT / "bin" / "kbsearch").exists(),
+            "bin/kbsearch nested module must be gone",
+        )
+
+    def test_no_main_go_under_bin_brain(self) -> None:
+        main = ROOT / "bin" / "brain" / "main.go"
+        self.assertFalse(main.exists(), "bin/brain/main.go is not a method")