docs(nixos): add runnable direnv + flake dev-shell examples

[GregHilston]

What & why

nixos/README.md already teaches our per-project dev-environment workflow — direnv + nix-direnv + a flake devShell, no Docker — but only as inline cat > flake.nix <<EOF heredocs pinned to aarch64-linux. Nothing you can actually cd into and run.

This adds two copy-and-run example projects under a new nixos/examples/ so the workflow is demonstrated by a real directory, not a snippet.

Changes

New nixos/examples/:

File	Purpose
README.md	Brief explainer: how direnv + flakes work, and the exact commands to run
.gitignore	Ignores .direnv/, __pycache__/, *.pyc, node_modules/, compiled hello.js
python-devshell/	flake.nix (Python 3 + requests + ruff), .envrc (use flake), hello.py
typescript-devshell/	flake.nix (Node.js 22 + typescript), .envrc (use flake), hello.ts

Both flakes use flake-utils.eachDefaultSystem, so they work on every host architecture with no edits — unlike the arch-pinned heredocs they complement.

Doc links (as requested):

• nixos/README.md — a pointer to examples/ added in the "Per-Project Development Environments" section (existing heredocs left in place).
• nixos/CLAUDE.md — one bullet under "File Locations".

Notes

• No committed flake.lock. The first direnv allow / nix develop generates one; the example README explains that a real project would commit it for byte-for-byte reproducibility.
• These are standalone flakes not referenced by the nixos flake, so nix flake check / the host dry-run builds never evaluate them — they're only text-formatted by treefmt.

Remaining / follow-up steps

These need a Nix host (this PR was authored in an environment without nix):

• Commit flake.lock for each example so contributors get a byte-for-byte identical toolchain:

  cd nixos/examples/python-devshell     && nix flake lock
  cd ../typescript-devshell             && nix flake lock
  git add nixos/examples/*/flake.lock && git commit -m "chore(examples): pin flake.lock"

• Verify each shell loads: cd nixos/examples/python-devshell && nix develop -c python hello.py, and cd nixos/examples/typescript-devshell && nix develop -c bash -c 'tsc hello.ts && node hello.js'.
• Confirm the fmt CI check (treefmt / alejandra) is green. If it flags the example flakes, run nix fmt nixos/ and commit — formatting couldn't be verified locally without nix.
• (Optional) direnv allow in each example dir to confirm auto-load-on-cd / unload-on-leave.

Verification (done in-session)

python -m py_compile hello.py passes; the flakes were hand-formatted to match the repo's alejandra style. Authoritative Nix checks are in the follow-up list above.

🤖 Generated with Claude Code

https://claude.ai/code/session_011XBAGAamSrYUHPoqXWL4rX