feat: rate limiting de tools por room en registry

Añade rate limiting de tool calls por room usando sliding window:

- tools/ratelimit.go: RateLimiter con sliding window per key (room),
  Allow() para verificar/registrar llamadas, Cleanup() para limpiar
  entries expiradas
- tools/registry.go: SetRateLimiter() y ExecuteForRoom() que verifica
  el rate limit antes de ejecutar, logueando tool_rate_limited si excede
- internal/config/schema.go: ToolRateLimitCfg en SecurityCfg con
  enabled, max_calls_per_min y cleanup_interval_s
- agents/runtime.go: inicializa rate limiter desde config y arranca
  goroutine de cleanup periodico
- agents/commands.go: usa ExecuteForRoom en !tool command

Config YAML:
  security:
    tool_rate_limit:
      enabled: true
      max_calls_per_min: 10

Parte de issue 0019c (prompt injection hardening — rate limiting).

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

tools/registry.go

@@ -14,8 +14,9 @@ import (
 
 // Registry holds available tools keyed by name.
 type Registry struct {
-	tools  map[string]Tool
-	logger *slog.Logger
+	tools       map[string]Tool
+	logger      *slog.Logger
+	rateLimiter *RateLimiter // nil when rate limiting is disabled
 }
 
 // NewRegistry creates an empty registry.
@@ -53,6 +54,24 @@ func (r *Registry) Len() int {
 	return len(r.tools)
 }
 
+// SetRateLimiter attaches a rate limiter to the registry.
+// When set, ExecuteForRoom checks the limit before running the tool.
+func (r *Registry) SetRateLimiter(rl *RateLimiter) {
+	r.rateLimiter = rl
+}
+
+// ExecuteForRoom is like Execute but checks the per-room rate limit first.
+// If the rate limit is exceeded, it returns an error result without executing.
+func (r *Registry) ExecuteForRoom(ctx context.Context, name, argsJSON, roomID string) Result {
+	if r.rateLimiter != nil && roomID != "" {
+		if !r.rateLimiter.Allow(roomID) {
+			r.logger.Warn("tool_rate_limited", "tool", name, "room", roomID)
+			return Result{Err: fmt.Errorf("rate limit exceeded for room %s: too many tool calls per minute", roomID)}
+		}
+	}
+	return r.Execute(ctx, name, argsJSON)
+}
+
 // Execute looks up a tool by name and runs it. Returns an error result if not found.
 func (r *Registry) Execute(ctx context.Context, name string, argsJSON string) Result {
 	t, ok := r.tools[name]