feat: añadir tools wikipedia_search y exchange_rate

- tools/wikipedia/wikipedia.go: tool wikipedia_search que consulta la
  API pública de Wikipedia (sin auth). Devuelve resumen del artículo.
- tools/exchange/exchange.go: 4 tools de tipo de cambio usando
  exchangerate-api.com: exchange_rate_get, exchange_rate_convert,
  exchange_rate_list, exchange_rate_historical.
- internal/config/schema.go: añadir ExchangeRateToolCfg con Enabled,
  APIKey, APIKeyEnv y Timeout.
- devagents/registry_build.go: registrar ambas tool families.
  wikipedia_search siempre disponible; exchange rate tools requieren
  APIKey configurado (deny-by-default con WARN si falta).
- devagents/registry_build_test.go: actualizar test de registry build.

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

internal/config/schema.go

@@ -164,6 +164,7 @@ type ToolsCfg struct {
 	SharedKnowledge SharedKnowledgeToolCfg `yaml:"shared_knowledge"`
 	Skills          SkillsToolCfg          `yaml:"skills"`
 	IMDb            IMDbToolCfg            `yaml:"imdb"`
+	ExchangeRate    ExchangeRateToolCfg    `yaml:"exchange_rate"`
 }
 
 type MatrixToolCfg struct {
@@ -438,6 +439,13 @@ type IMDbToolCfg struct {
 	Timeout   time.Duration `yaml:"timeout"`     // timeout for API requests (default: 10s)
 }
 
+type ExchangeRateToolCfg struct {
+	Enabled   bool          `yaml:"enabled"`
+	APIKey    string        `yaml:"api_key"`     // ExchangeRate-API key (get from https://www.exchangerate-api.com)
+	APIKeyEnv string        `yaml:"api_key_env"` // env var name for API key (e.g., "EXCHANGE_RATE_API_KEY")
+	Timeout   time.Duration `yaml:"timeout"`     // timeout for API requests (default: 10s)
+}
+
 // ── Special Agents ────────────────────────────────────────────────────────
 
 // SpecialConfig is the root configuration for a special agent (no Matrix identity).