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

AGENTS.md

@@ -3,8 +3,9 @@
 ## Toolchain (mise)
 
 ```bash
-mise trust      # trust the mise.toml config (first time only)
-mise install    # installs go 1.26, node 22
+mise trust        # trust the mise.toml config (first time only)
+mise install      # installs go 1.26, node 22
+mise run gopls    # install gopls (Go LSP) — one-time dev setup
 ```
 
 All build/run tasks are defined in `mise.toml`: