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.

internal/brain/modeldir.go

@@ -0,0 +1,59 @@
+package brain
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+)
+
+func modelDir() (string, error) {
+	// 1. Explicit env
+	if v := os.Getenv("KBSEARCH_MODEL"); v != "" {
+		return v, nil
+	}
+	// 2. Next to the binary (dev or installed)
+	self, err := os.Executable()
+	if err == nil {
+		if dir, err := filepath.EvalSymlinks(filepath.Dir(self)); err == nil {
+			cand := filepath.Join(dir, "potion-multilingual-128m")
+			if st, err := os.Stat(cand); err == nil && st.IsDir() {
+				return cand, nil
+			}
+		}
+	}
+	// 3. Repo root lib/ (where other scripts expect it)
+	if v := os.Getenv("KB_ROOT"); v != "" {
+		cand := filepath.Join(v, "lib", "potion-multilingual-128m")
+		if st, err := os.Stat(cand); err == nil && st.IsDir() {
+			return cand, nil
+		}
+	}
+	// 4. HF cache (new layout: models--*/snapshots/*)
+	if v := os.Getenv("HF_HOME"); v != "" {
+		base := filepath.Join(v, "hub")
+		if entries, err := os.ReadDir(base); err == nil {
+			for _, e := range entries {
+				if strings.HasPrefix(e.Name(), "models--") {
+					snapDir := filepath.Join(base, e.Name(), "snapshots")
+					if snaps, err := os.ReadDir(snapDir); err == nil {
+						for _, s := range snaps {
+							cand := filepath.Join(snapDir, s.Name())
+							if st, _ := os.Stat(cand); st != nil && st.IsDir() {
+								return cand, nil
+							}
+						}
+					}
+				}
+			}
+		}
+	}
+	// 5. Legacy HF cache (symlinked model dir)
+	if v := os.Getenv("HF_HOME"); v != "" {
+		cand := filepath.Join(v, "potion-multilingual-128m")
+		if st, err := os.Stat(cand); err == nil && st.IsDir() {
+			return cand, nil
+		}
+	}
+	return "", fmt.Errorf("model not found (set KBSEARCH_MODEL or KB_ROOT, or download to HF cache)")
+}