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/rank/args.go

@@ -0,0 +1,72 @@
+package rank
+
+import (
+	"fmt"
+	"strconv"
+	"strings"
+)
+
+const Usage = `usage: bin/brain/search.go "query" [--root facts|info] [--repo REPO] [-n N] [--json]
+       bin/brain/search.go serve [port]
+       bin/brain/search.go --list-model`
+
+type Options struct {
+	Query     string
+	Root      string
+	Repo      string
+	Limit     int
+	JSONOut   bool
+	ListModel bool
+}
+
+// ParseArgs reads flags. Unknown flags are an error: silently dropping them
+// meant `--hop 1` vanished and its argument `1` was appended to the query.
+// --hop is recognised so it cannot be swallowed; it is not implemented until
+// File/FROM_FILE edges exist.
+func ParseArgs(args []string) (Options, error) {
+	opt := Options{Limit: 20}
+	var queryArgs []string
+
+	for i := 0; i < len(args); i++ {
+		arg := args[i]
+		wantsValue := arg == "--root" || arg == "--repo" || arg == "-n" || arg == "--hop"
+		if wantsValue && i+1 >= len(args) {
+			return opt, fmt.Errorf("%s needs a value", arg)
+		}
+		switch arg {
+		case "--root":
+			i++
+			opt.Root = args[i]
+			if opt.Root != "facts" && opt.Root != "info" {
+				return opt, fmt.Errorf("--root must be facts or info, got %q", opt.Root)
+			}
+		case "--repo":
+			i++
+			opt.Repo = args[i]
+		case "-n":
+			i++
+			n, err := strconv.Atoi(args[i])
+			if err != nil || n < 1 {
+				return opt, fmt.Errorf("-n must be a positive integer, got %q", args[i])
+			}
+			opt.Limit = n
+		case "--hop":
+			return opt, fmt.Errorf("--hop is not implemented yet (needs File/FROM_FILE edges)")
+		case "--json":
+			opt.JSONOut = true
+		case "--list-model":
+			opt.ListModel = true
+		default:
+			if strings.HasPrefix(arg, "-") {
+				return opt, fmt.Errorf("unknown flag %q", arg)
+			}
+			queryArgs = append(queryArgs, arg)
+		}
+	}
+
+	opt.Query = strings.TrimSpace(strings.Join(queryArgs, " "))
+	if opt.Query == "" && !opt.ListModel {
+		return opt, fmt.Errorf("no query given")
+	}
+	return opt, nil
+}