feat: soporte de threads de Matrix (m.thread)

Implementa el soporte completo de threads de Matrix:
- Listener extrae ThreadID de m.relates_to con rel_type=m.thread
- Client.SendThreadMarkdown envia mensajes como parte de un thread
  usando SetThread de mautrix con fallback m.in_reply_to
- Runner detecta ThreadID en ReplyAction y rutea a SendThreadMarkdown
- MatrixSender interfaz actualizada con SendThreadMarkdown
- runtime.go propaga ThreadID en todas las respuestas (comandos, LLM, RBAC)
- sendReply helper centraliza la logica de envio con/sin thread
- Auto-thread: si matrix.threads.auto_thread=true, crea thread nuevo
  para cada conversacion que no esta ya en un thread
- Memoria por thread: usa ThreadID como clave de window cuando el mensaje
  esta en un thread, permitiendo conversaciones paralelas independientes
- Config: matrix.threads.enabled y matrix.threads.auto_thread en ThreadsCfg

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

shell/matrix/client.go

@@ -320,6 +320,26 @@ func (c *Client) SendReplyMarkdown(ctx context.Context, roomID, inReplyTo, markd
 	return err
 }
 
+// SendThreadMarkdown sends a formatted message as part of a Matrix thread.
+// threadRootID is the event that started the thread (always the same for all messages in a thread).
+// inReplyTo is the specific event being replied to within the thread (used as fallback for non-thread clients).
+// If inReplyTo is empty, it defaults to threadRootID.
+func (c *Client) SendThreadMarkdown(ctx context.Context, roomID, threadRootID, inReplyTo, markdown string) error {
+	if inReplyTo == "" {
+		inReplyTo = threadRootID
+	}
+	html := mdToHTML(markdown)
+	content := event.MessageEventContent{
+		MsgType:       event.MsgText,
+		Body:          markdown,
+		Format:        event.FormatHTML,
+		FormattedBody: html,
+		RelatesTo:     (&event.RelatesTo{}).SetThread(id.EventID(threadRootID), id.EventID(inReplyTo)),
+	}
+	_, err := c.raw.SendMessageEvent(ctx, id.RoomID(roomID), event.EventMessage, content)
+	return err
+}
+
 // SendReaction sends a reaction to an event.
 func (c *Client) SendReaction(ctx context.Context, roomID, eventID, reaction string) error {
 	_, err := c.raw.SendReaction(ctx, id.RoomID(roomID), id.EventID(eventID), reaction)