feat: add Exa AI search provider

Add Exa (https://exa.ai) as a new web search provider option, slotting
into the priority chain between Perplexity and Brave. Configurable via
config.json or PICOCLAW_TOOLS_WEB_EXA_* environment variables.

Results are capped to the requested count for consistency with other
search providers.

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

pkg/config/config.go

@@ -530,11 +530,18 @@ type PerplexityConfig struct {
 	MaxResults int    `json:"max_results" env:"PICOCLAW_TOOLS_WEB_PERPLEXITY_MAX_RESULTS"`
 }
 
+type ExaConfig struct {
+	Enabled    bool   `json:"enabled"     env:"PICOCLAW_TOOLS_WEB_EXA_ENABLED"`
+	APIKey     string `json:"api_key"     env:"PICOCLAW_TOOLS_WEB_EXA_API_KEY"`
+	MaxResults int    `json:"max_results" env:"PICOCLAW_TOOLS_WEB_EXA_MAX_RESULTS"`
+}
+
 type WebToolsConfig struct {
 	Brave      BraveConfig      `json:"brave"`
 	Tavily     TavilyConfig     `json:"tavily"`
 	DuckDuckGo DuckDuckGoConfig `json:"duckduckgo"`
 	Perplexity PerplexityConfig `json:"perplexity"`
+	Exa        ExaConfig        `json:"exa"`
 	// Proxy is an optional proxy URL for web tools (http/https/socks5/socks5h).
 	// For authenticated proxies, prefer HTTP_PROXY/HTTPS_PROXY env vars instead of embedding credentials in config.
 	Proxy           string `json:"proxy,omitempty"             env:"PICOCLAW_TOOLS_WEB_PROXY"`

pkg/config/defaults.go

@@ -341,6 +341,11 @@ func DefaultConfig() *Config {
 					APIKey:     "",
 					MaxResults: 5,
 				},
+				Exa: ExaConfig{
+					Enabled:    false,
+					APIKey:     "",
+					MaxResults: 5,
+				},
 			},
 			Cron: CronToolsConfig{
 				ExecTimeoutMinutes: 5,