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/db.go

@@ -0,0 +1,100 @@
+//go:build cgo && system_ladybug
+
+package brain
+
+import (
+	"fmt"
+	"os"
+	"path/filepath"
+	"strings"
+
+	lbug "github.com/LadybugDB/go-ladybug"
+)
+
+var (
+	db   *lbug.Database
+	conn *lbug.Connection
+)
+
+func repoRoot() string {
+	// Try KB_ROOT env, then walk up from binary
+	if v := os.Getenv("KB_ROOT"); v != "" {
+		return v
+	}
+	self, err := os.Executable()
+	if err == nil {
+		dir := filepath.Dir(self)
+		for i := 0; i < 5; i++ {
+			if _, err := os.Stat(filepath.Join(dir, "var")); err == nil {
+				return dir
+			}
+			if _, err := os.Stat(filepath.Join(dir, ".git")); err == nil {
+				return dir
+			}
+			parent := filepath.Dir(dir)
+			if parent == dir {
+				break
+			}
+			dir = parent
+		}
+	}
+	return "."
+}
+
+func dbPath() string {
+	return filepath.Join(repoRoot(), "var", "kb.lbug")
+}
+
+func openBrain() error {
+	return openWithSandbox(eps())
+}
+
+func openWithSandbox(epsv string) error {
+	cfg := lbug.DefaultSystemConfig()
+	cfg.MaxNumThreads = 8
+	cfg.BufferPoolSize = 1 << 30 // 1GB
+
+	var err error
+	db, err = lbug.OpenDatabase(dbPath(), cfg)
+	if err != nil {
+		return fmt.Errorf("OpenDatabase: %w", err)
+	}
+
+	conn, err = lbug.OpenConnection(db)
+	if err != nil {
+		closeBrain()
+		return fmt.Errorf("OpenConnection: %w", err)
+	}
+	// Session settings need a live connection; running this before
+	// OpenConnection dereferenced a nil *Connection.
+	if epsv != "" {
+		if strings.ContainsAny(epsv, "'\\") {
+			closeBrain()
+			return fmt.Errorf("SET STREAM_SANDBOX: invalid value")
+		}
+		if _, err := conn.Query("SET STREAM_SANDBOX = '" + epsv + "'"); err != nil {
+			closeBrain()
+			return fmt.Errorf("SET STREAM_SANDBOX: %w", err)
+		}
+	}
+	if _, err := conn.Query("LOAD EXTENSION FTS"); err != nil {
+		closeBrain()
+		return fmt.Errorf("LOAD EXTENSION FTS: %w", err)
+	}
+	if _, err := conn.Query("LOAD EXTENSION VECTOR"); err != nil {
+		closeBrain()
+		return fmt.Errorf("LOAD EXTENSION VECTOR: %w", err)
+	}
+	return nil
+}
+
+func closeBrain() {
+	if conn != nil {
+		conn.Close()
+		conn = nil
+	}
+	if db != nil {
+		db.Close()
+		db = nil
+	}
+}