chore: stop tracking .envrc

[AlexHardAtWork]

What

Stop tracking .envrc and add it to .gitignore. The file stays on disk — only the
tracking is dropped.

Why

.envrc is per-developer environment config, but it is committed, so every local
adjustment (a DIRENV_WARN_TIMEOUT, an extra export, a dotenv line) shows up as a dirty
worktree and can be committed by accident.

It is also inconsistent with how the repo already handles environment config: .env is
gitignored and .env.example is the tracked template — cp .env.example .env is step one
of setup in both README.md and CONTRIBUTING.md. .envrc is the one env file that
doesn't follow that rule.

For contributors

Pulling this deletes your local .envrc, since git removes the file it no longer tracks.
Restore it once and it will stay:

printf '#!/usr/bin/env bash\n\nexport DIRENV_WARN_TIMEOUT=20s\n\nuse flake\n' > .envrc
direnv allow

If we would rather keep a starting point in the repo, a follow-up can add .envrc.example
alongside .env.example and document it in CONTRIBUTING.md.