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.

context/download.go

@@ -0,0 +1,50 @@
+package context
+
+import (
+	stdctx "context"
+	"fmt"
+
+	"go.mau.fi/whatsmeow"
+	waE2E "go.mau.fi/whatsmeow/proto/waE2E"
+)
+
+func (c *Ctx) Download() ([]byte, error) {
+	msg := getDownloadable(c.message)
+	if msg == nil {
+		return nil, fmt.Errorf("message has no downloadable media")
+	}
+	return c.client.Download(stdctx.Background(), msg)
+}
+
+func (c *Ctx) DownloadQuoted() ([]byte, error) {
+	quoted := GetQuotedMessage(c.event.Message)
+	if quoted == nil {
+		return nil, fmt.Errorf("no quoted message")
+	}
+	msg := getDownloadable(quoted)
+	if msg == nil {
+		return nil, fmt.Errorf("quoted message has no downloadable media")
+	}
+	return c.client.Download(stdctx.Background(), msg)
+}
+
+func (c *Ctx) DownloadMessage(msg whatsmeow.DownloadableMessage) ([]byte, error) {
+	return c.client.Download(stdctx.Background(), msg)
+}
+
+func getDownloadable(m *waE2E.Message) whatsmeow.DownloadableMessage {
+	switch {
+	case m.GetImageMessage() != nil:
+		return m.GetImageMessage()
+	case m.GetVideoMessage() != nil:
+		return m.GetVideoMessage()
+	case m.GetAudioMessage() != nil:
+		return m.GetAudioMessage()
+	case m.GetDocumentMessage() != nil:
+		return m.GetDocumentMessage()
+	case m.GetStickerMessage() != nil:
+		return m.GetStickerMessage()
+	default:
+		return nil
+	}
+}