docs: add nix develop note to CLAUDE.md

Go commands should be run via nix develop -c to ensure the proper
build environment with all dependencies (like gcc for cgo).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

CLAUDE.md

@@ -183,3 +183,5 @@ labmcp/
 - Use `database/sql` interface for database abstraction
 - File paths in responses should use format `path/to/file.go:123`
 - **Always run `go fmt ./...` before committing Go code**
+- **Run Go commands using `nix develop -c`** (e.g., `nix develop -c go test ./...`) to ensure proper build environment with all dependencies
+- **Use `nix run` to run binaries** instead of `go build` followed by running the binary (e.g., `nix run .#nixos-options -- serve`)