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

scripts/check-toolchain.sh

@@ -98,6 +98,19 @@ fi
 
 echo ""
 
+# ─── gopls (Go LSP) ────────────────────────────────────────────────────
+echo "gopls (Go language server):"
+if command -v gopls &>/dev/null; then
+    gopls_ver=$(gopls version 2>/dev/null)
+    ok "gopls found: $gopls_ver"
+else
+    warn "gopls not found (recommended for IDE support)"
+    echo "    Install: go install golang.org/x/tools/gopls@latest"
+    echo "    (ensure $(go env GOPATH)/bin is in PATH)"
+fi
+
+echo ""
+
 # ─── Node ──────────────────────────────────────────────────────────────
 echo "Node:"
 if command -v node &>/dev/null; then