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

main.go

@@ -250,7 +250,9 @@ func cmdDownload(args []string) {
 	jobs := fs.Int("jobs", 2, "concurrent downloads")
 	_ = fs.Parse(args)
 
-	os.MkdirAll(*outDir, 0755)
+	if err := os.MkdirAll(*outDir, 0755); err != nil {
+		fatal("create %s: %v", *outDir, err)
+	}
 
 	var identifiers []string
 	if *identifier != "" {
@@ -383,7 +385,9 @@ func cmdExtract(args []string) {
 	jobs := fs.Int("jobs", 2, "concurrent extractions")
 	_ = fs.Parse(args)
 
-	os.MkdirAll(*outDir, 0755)
+	if err := os.MkdirAll(*outDir, 0755); err != nil {
+		fatal("create %s: %v", *outDir, err)
+	}
 
 	entries, err := os.ReadDir(*isoDir)
 	if err != nil {