feat: hot-reload de agentes individuales via SIGHUP

Implementa el mecanismo de hot-reload descrito en el issue 0013:

- agents/runtime.go: añade Agent.Stop() y Agent.Done() para ciclo de vida
  individual. Run() crea un contexto hijo cancelable y cierra el canal
  done al retornar.

- cmd/launcher/registry.go (nuevo): agentRegistry rastrea agentes vivos
  por ID. Métodos: register, stopAndWait, reload, reloadAll, waitAll,
  cleanupLogs. reload() sigue el flujo completo: stop→wait→unsubscribe
  →reload config→recreate→rewire bus/orch→start nueva goroutine.

- cmd/launcher/main.go: usa agentRegistry en lugar de sync.WaitGroup.
  Añade handler de SIGHUP en goroutine separada que lee run/reload.txt
  para determinar el agente objetivo (* o vacío = todos). Tras leer,
  borra run/reload.txt para no afectar el siguiente SIGHUP.

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

agents/runtime.go

@@ -62,6 +62,10 @@ type Agent struct {
 	logger      *slog.Logger
 	cryptoStore io.Closer // non-nil when E2EE is enabled; closed on shutdown
 
+	// Lifecycle — cancel stops this agent individually; done is closed when Run returns.
+	cancel context.CancelFunc
+	done   chan struct{}
+
 	// Access control
 	acl acl.ACL
 
@@ -268,6 +272,7 @@ func New(cfg *config.AgentConfig, rules []decision.Rule, logger *slog.Logger) (*
 		toolReg:     toolReg,
 		logger:      logger,
 		cryptoStore: cryptoStore,
+		done:           make(chan struct{}),
 		commands:       make(map[string]CommandHandler),
 		cmdAliases:     command.BuiltinNames(),
 		startTime:      time.Now(),
@@ -363,8 +368,24 @@ func (a *Agent) RawMatrixClient() *mautrix.Client {
 	return a.matrix.Raw()
 }
 
+// Stop cancels this agent's individual context, causing Run to return.
+// Safe to call multiple times.
+func (a *Agent) Stop() {
+	if a.cancel != nil {
+		a.cancel()
+	}
+}
+
+// Done returns a channel that is closed when Run has returned.
+func (a *Agent) Done() <-chan struct{} {
+	return a.done
+}
+
 // Run starts the agent sync loop. Blocks until ctx is cancelled.
 func (a *Agent) Run(ctx context.Context) error {
+	ctx, a.cancel = context.WithCancel(ctx)
+	defer close(a.done)
+
 	if a.cryptoStore != nil {
 		defer a.cryptoStore.Close()
 	}