fix: check MkdirAll errors, add gopls to toolchain

- serve.go/cmdAPIDownload: check MkdirAll error, use absolute path
- main.go/cmdDownload,cmdExtract: check MkdirAll errors
- check-toolchain.sh: verify gopls installation
- mise.toml: add gopls install task
- AGENTS.md: document gopls as dev dependency

serve.go

@@ -158,7 +158,13 @@ func handleAPIDownload(w http.ResponseWriter, r *http.Request) {
 		outDir = "."
 	}
 
-	os.MkdirAll(outDir, 0755)
+	if abs, err := filepath.Abs(outDir); err == nil {
+		outDir = abs
+	}
+	if err := os.MkdirAll(outDir, 0755); err != nil {
+		writeJSON(w, map[string]any{"error": fmt.Sprintf("create %s: %v", outDir, err)})
+		return
+	}
 
 	files := getItemFiles(identifier)
 	if len(files) == 0 {
@@ -287,8 +293,6 @@ func handleAPIExtract(w http.ResponseWriter, r *http.Request) {
 		outDir = "./wavs"
 	}
 
-	os.MkdirAll(outDir, 0755)
-
 	entries, err := os.ReadDir(isoDir)
 	if err != nil {
 		writeJSON(w, map[string]any{"error": err.Error()})