fix(config): add missing Tavily default config in defaults.go

The TavilyConfig struct has BaseURL field, but the default config in
defaults.go was missing the entire Tavily configuration. This adds the
complete default config for Tavily with all fields initialized to their
zero values.

pkg/config/defaults.go

@@ -286,6 +286,12 @@ func DefaultConfig() *Config {
 					Enabled:    true,
 					MaxResults: 5,
 				},
+				Tavily: TavilyConfig{
+					Enabled:    false,
+					APIKey:     "",
+					BaseURL:    "",
+					MaxResults: 5,
+				},
 				Perplexity: PerplexityConfig{
 					Enabled:    false,
 					APIKey:     "",