feat: sistema RegisterCommand para comandos por agente

Añade RegisterCommand(spec, handler) al Agent para que cada agente pueda
registrar comandos propios (!xxx) sin modificar built-ins ni usar reglas.

Cambios principales:
- agents/runtime.go: nuevo método RegisterCommand + campo customSpecs
- agents/commands.go: !help muestra comandos del agente en sección separada,
  extrae writeSpec helper, elimina customCommandRules (ya no se usan reglas
  para comandos)
- handleEvent simplificado: los comandos se resuelven por lookup directo
  (built-in + registered), nunca pasan por reglas ni LLM

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

agents/commands.go

@@ -22,7 +22,7 @@ func (a *Agent) registerBuiltinCommands() {
 	a.commands["version"] = a.cmdVersion
 }
 
-// cmdHelp lists all available commands (built-in + custom rules with MatchCommand).
+// cmdHelp lists all available commands (built-in + agent-specific).
 func (a *Agent) cmdHelp(_ context.Context, _ decision.MessageContext) string {
 	var b strings.Builder
 	b.WriteString("Comandos disponibles:\n\n")
@@ -32,25 +32,36 @@ func (a *Agent) cmdHelp(_ context.Context, _ decision.MessageContext) string {
 		if spec.Hidden {
 			continue
 		}
-		aliases := ""
-		if len(spec.Aliases) > 0 {
-			aliases = " (" + strings.Join(prefixAll(spec.Aliases, "!"), ", ") + ")"
-		}
-		fmt.Fprintf(&b, "  %s%s — %s\n", spec.Usage, aliases, spec.Description)
+		writeSpec(&b, spec)
 	}
 
-	// Custom commands from agent rules (rules named with MatchCommand pattern)
-	customRules := a.customCommandRules()
-	if len(customRules) > 0 {
+	// Agent-specific commands (registered via RegisterCommand)
+	if len(a.customSpecs) > 0 {
 		b.WriteString("\nComandos del agente:\n")
-		for _, name := range customRules {
-			fmt.Fprintf(&b, "  !%s\n", name)
+		for _, spec := range a.customSpecs {
+			if spec.Hidden {
+				continue
+			}
+			writeSpec(&b, spec)
 		}
 	}
 
 	return b.String()
 }
 
+// writeSpec formats a single command spec for the help output.
+func writeSpec(b *strings.Builder, spec command.Spec) {
+	aliases := ""
+	if len(spec.Aliases) > 0 {
+		aliases = " (" + strings.Join(prefixAll(spec.Aliases, "!"), ", ") + ")"
+	}
+	usage := spec.Usage
+	if usage == "" {
+		usage = "!" + spec.Name
+	}
+	fmt.Fprintf(b, "  %s%s — %s\n", usage, aliases, spec.Description)
+}
+
 // cmdTools lists all tools registered in the agent's tool registry.
 func (a *Agent) cmdTools(_ context.Context, _ decision.MessageContext) string {
 	names := a.toolReg.Names()
@@ -159,17 +170,6 @@ func (a *Agent) cmdVersion(_ context.Context, _ decision.MessageContext) string
 	return fmt.Sprintf("%s %s", a.cfg.Agent.Name, v)
 }
 
-// customCommandRules extracts rule names that look like command handlers.
-func (a *Agent) customCommandRules() []string {
-	var names []string
-	for _, r := range a.rules {
-		if r.Name != "" {
-			names = append(names, r.Name)
-		}
-	}
-	return names
-}
-
 // prefixAll adds a prefix to each string in a slice.
 func prefixAll(ss []string, prefix string) []string {
 	out := make([]string, len(ss))