feat: initial whatsmeow bot with command handler and context helpers

Core framework (Handler, Command, Default muxer) in core/ package.
Command self-registration via init() with blank imports in cmds/.
Context helpers (send, reply, download, upload) in context/ package.
Permission system with GROUP_ONLY / PRIVATE_ONLY checks.
WhatsApp client configured as Safari with history sync disabled.

core/permission.go

@@ -0,0 +1,45 @@
+package core
+
+import (
+	"context"
+
+	"go.mau.fi/whatsmeow"
+	waE2E "go.mau.fi/whatsmeow/proto/waE2E"
+	"go.mau.fi/whatsmeow/types/events"
+	"google.golang.org/protobuf/proto"
+)
+
+type Permission string
+
+const (
+	PermissionGroupOnly   Permission = "GROUP_ONLY"
+	PermissionPrivateOnly Permission = "PRIVATE_ONLY"
+)
+
+// CheckCommandPermissions returns an error message if the command's
+// permissions deny the event context, or empty string if allowed.
+func CheckCommandPermissions(cmd *Command, evt *events.Message) string {
+	for _, p := range cmd.Permissions {
+		switch p {
+		case PermissionGroupOnly:
+			if !evt.Info.IsGroup {
+				return "This command can only be used in groups"
+			}
+		case PermissionPrivateOnly:
+			if evt.Info.IsGroup {
+				return "This command can only be used in private chat"
+			}
+		}
+	}
+	return ""
+}
+
+// sendPermissionDenied sends a plain text reply indicating why the
+// command was blocked, so the caller gets feedback instead of silence.
+func sendPermissionDenied(client *whatsmeow.Client, evt *events.Message, text string) {
+	client.SendMessage(context.Background(), evt.Info.Chat, &waE2E.Message{
+		ExtendedTextMessage: &waE2E.ExtendedTextMessage{
+			Text: proto.String(text),
+		},
+	})
+}