make direnv opt-in via sample files + local dotenv support

[edvardm]

Committed .envrc enforces use nix; too opinionated. Perhaps we could use this as recommended .envrc entry without enforcing it

Proposal

• Move sample env files to an assets/resources dir (e.g., assets/sample/).
• Add .setup-sample-files task to copy samples only if targets don’t exist.
• Stop tracking .envrc in git.
• Sample .envrc should source .envrc.local and dotenv_if_exists .env.

Acceptance

• Setup preserves existing local files; no overwrites.
• Existing .envrc users get equivalent behavior after revised setup.
• .envrc.local and .env are optional and supported out of the box.