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/agent/loop.go

@@ -112,6 +112,9 @@ func registerSharedTools(
 			PerplexityAPIKey:     cfg.Tools.Web.Perplexity.APIKey,
 			PerplexityMaxResults: cfg.Tools.Web.Perplexity.MaxResults,
 			PerplexityEnabled:    cfg.Tools.Web.Perplexity.Enabled,
+			ExaAPIKey:            cfg.Tools.Web.Exa.APIKey,
+			ExaMaxResults:        cfg.Tools.Web.Exa.MaxResults,
+			ExaEnabled:           cfg.Tools.Web.Exa.Enabled,
 			Proxy:                cfg.Tools.Web.Proxy,
 		})
 		if err != nil {