fix: address code review (v3/v2 detect, gemini caps, fetch timeout, audio mime, json extract, dead temp config)

- detect: recaptcha_v3 now suppresses recaptcha_v2 (hidden g-recaptcha div on v3 pages no longer mis-detected as LLM-solvable); funcaptcha drops over-broad bare "arkose" substring
- llm/gemini: stub methods return ErrUnsupported{Provider:"gemini"} so audio handler errors.As guard catches it; drop unused fmt import
- mcp/tools: remove gemini from audio capability (stub); pick audio filename (.wav/.mp3) from sniffed media type; schema notes for browser/actions no longer advertise unimplemented options
- imageutil: fetchURL uses a package client with 30s Timeout instead of http.DefaultClient
- captcha/json: extractJSON brace-counts to return the first balanced object (handles prose, markdown fences, example-before-real)
- config: remove dead CAPTCHA_LLM_TEMPERATURE / Config.Temperature; solving is fixed at temperature 0
- stealth: Action.Type doc comment drops nonexistent scroll case

internal/captcha/detect.go

@@ -21,8 +21,7 @@ func DetectHTML(html string) []Detection {
 			Type: "recaptcha_v3", Sitekey: m[1], LLMSolvable: false,
 			Notes: "score/behavioral — not LLM-solvable; needs browser reputation",
 		})
-	}
-	if strings.Contains(html, "g-recaptcha") || strings.Contains(html, "google.com/recaptcha/api2") {
+	} else if strings.Contains(html, "g-recaptcha") || strings.Contains(html, "google.com/recaptcha/api2") {
 		out = append(out, Detection{Type: "recaptcha_v2", Sitekey: firstSitekey(html), LLMSolvable: true,
 			Notes: "image-grid; LLM returns tiles, caller clicks + harvests token"})
 	}
@@ -34,7 +33,7 @@ func DetectHTML(html string) []Detection {
 		out = append(out, Detection{Type: "turnstile", Sitekey: firstSitekey(html), LLMSolvable: false,
 			Notes: "proof-of-work/behavioral — not LLM-solvable"})
 	}
-	if strings.Contains(html, "arkoselabs") || strings.Contains(html, "funcaptcha") || strings.Contains(html, "arkose") {
+	if strings.Contains(html, "arkoselabs") || strings.Contains(html, "funcaptcha") {
 		out = append(out, Detection{Type: "funcaptcha", Sitekey: firstPkey(html), LLMSolvable: true,
 			Notes: "rotation/visual — best-effort"})
 	}