fix: add goroutine safety to Discord typing indicator

- Add 5-minute timeout as safety net to prevent indefinite goroutine leaks
  when agent produces no outbound message (empty response, panic, etc.)
- Listen on c.ctx.Done() so goroutine exits when channel context is cancelled
- Log ChannelTyping() errors at debug level for diagnostics (rate limits, session closed)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

pkg/channels/discord.go

@@ -295,15 +295,24 @@ func (c *DiscordChannel) startTyping(chatID string) {
 	c.typingMu.Unlock()
 
 	go func() {
-		c.session.ChannelTyping(chatID)
+		if err := c.session.ChannelTyping(chatID); err != nil {
+			logger.DebugCF("discord", "ChannelTyping error", map[string]interface{}{"chatID": chatID, "err": err})
+		}
 		ticker := time.NewTicker(8 * time.Second)
 		defer ticker.Stop()
+		timeout := time.After(5 * time.Minute)
 		for {
 			select {
 			case <-stop:
 				return
+			case <-timeout:
+				return
+			case <-c.ctx.Done():
+				return
 			case <-ticker.C:
-				c.session.ChannelTyping(chatID)
+				if err := c.session.ChannelTyping(chatID); err != nil {
+					logger.DebugCF("discord", "ChannelTyping error", map[string]interface{}{"chatID": chatID, "err": err})
+				}
 			}
 		}
 	}()