Set up Cursor Cloud dev environment for Ember

[besmpl]

Summary

Sets up the development environment for the Ember Luau-compatible runtime and documents durable, non-obvious notes for future cloud agents.

• Installed Go 1.26.5 (the module pins go 1.26 in go.mod; the environment shipped with go1.22 and GOTOOLCHAIN auto-download of 1.26 does not work here). Go 1.26 lives at /usr/local/go and is first on PATH.
• Added a ## Cursor Cloud specific instructions section to AGENTS.md capturing the toolchain caveat, the fact there are no external module deps, and how to exercise the library (no server/CLI — use the public Compile/Run API).
• Update script set to go mod download (minimal, idempotent; effectively a no-op given no external deps).

Verification

• go build ./... — OK
• scripts/check (gofmt + shell syntax + go test ./... + git diff --check) — passing (~25s)
• Hello-world via the public API (compile + run) exercising arithmetic, the table base library, and a host-global Go callback:

arithmetic -> [7]
tables     -> [2, "hello world"]
host call  -> ["Hello, Ember!"]

No source code was modified; the only repo change is the AGENTS.md documentation addition.