Refactor once with astcount

Make one cohesive refactor that reduces Tree-sitter named-node count without changing behavior.

Measure

Infer the measurement scope from the request, defaulting to the repository.

Resolve one astcount command before measuring, in this order:

1. Use astcount when it is already installed.
2. Otherwise, use bunx astcount@0.2.0 when Bun is available.
3. Otherwise, use nix run github:wokalski/astcount/v0.2.0 -- when Nix is available.
4. If none is available, stop and tell the user that Bun or Nix is required.