feat(channels/buzz): implement ReactionCapable for 👀 emoji reactions

Add ReactToMessage method that publishes a NIP-25 kind:7 reaction event
with content 👀, scoped to the same channel via the h tag and pointing at
the inbound message via the e tag. The undo function is a no-op since
NIP-25 does not define a standard way to remove a reaction.

This enables the BaseChannel auto-reaction pipeline: when a message
arrives on a Buzz channel, the bot reacts with 👀 before processing,
giving the Emperor visual confirmation that the message was received.

pkg/channels/buzz/buzz.go

@@ -280,6 +280,47 @@ func (c *BuzzChannel) Send(ctx context.Context, msg bus.OutboundMessage) ([]stri
 	return []string{evt.ID}, nil
 }
 
+// kindReaction is the NIP-25 reaction event kind.
+const kindReaction = 7
+
+// ReactToMessage implements channels.ReactionCapable.
+// It publishes a NIP-25 kind:7 reaction event with content 👀 scoped to the
+// same channel as the reacted message. The undo function is a no-op because
+// NIP-25 does not define a standard way to remove a reaction.
+func (c *BuzzChannel) ReactToMessage(ctx context.Context, chatID, messageID string) (func(), error) {
+	if !c.IsRunning() {
+		return func() {}, channels.ErrNotRunning
+	}
+	if messageID == "" {
+		return func() {}, nil
+	}
+
+	evt := nostr.Event{
+		PubKey:    c.publicKey,
+		CreatedAt: nostr.Now(),
+		Kind:      kindReaction,
+		Tags: nostr.Tags{
+			nostr.Tag{"e", messageID},
+			nostr.Tag{"h", chatID},
+		},
+		Content: "👀",
+	}
+	if err := evt.Sign(c.secretKey); err != nil {
+		return func() {}, fmt.Errorf("buzz reaction sign failed: %w", err)
+	}
+
+	if err := c.relay.Publish(ctx, evt); err != nil {
+		return func() {}, fmt.Errorf("buzz reaction publish failed: %w", err)
+	}
+
+	logger.DebugCF("buzz", "Reaction sent", map[string]any{
+		"channel":   chatID,
+		"event_id":  evt.ID,
+		"target_id": messageID,
+	})
+	return func() {}, nil
+}
+
 // errJoin wraps err so that errors.Is(result, sentinel) holds for the sentinel
 // while preserving the underlying cause in the message.
 func errJoin(err, sentinel error) error {