feat: add Nix flake dev shell

[DanielMontilla]

What does this change, and why?

Adds a Nix flake dev shell so contributors on Nix can skip the manual toolchain setup from CONTRIBUTING.md. From the repo root, nix develop drops into a shell with Node 24, Python 3.12, Go 1.26.6, goreleaser, git, and jq pre-installed, and prints a banner with the resolved versions of each.

Tooling + docs only: no spec/schema changes (the frozen 1.0 line is untouched), no SDK behavior changes.

Checklist

• Every commit is signed off (git commit -s), and I agree to the Contributor terms
• N/A — no schemas/templates/cli.json changes (npm run assets:check unaffected)
• N/A — no behavior changes, so no SDK parity work
• Tests: nix develop shell verified locally; three suites unaffected by this change
• N/A — no spec/schema changes
• N/A — no context-layer delta