fix: make .envrc compatible with nix-direnv, allowing shell caching

[fguery]

Summary

• Update use flake --impure to use flake . --impure in .envrc
• This makes the flake command compatible with nix-direnv, which requires an explicit flake reference to enable shell caching
• Without the . argument, nix-direnv falls back to nix-shell behavior and cannot cache the dev shell

Test plan

• Verify direnv allow works correctly with the updated .envrc
• Confirm nix-direnv caches the shell properly

🤖 Generated with Claude Code

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ℹ️ Review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: a6fa3c49-92d5-4c5e-80fe-734cda82ca41

📥 Commits

Reviewing files that changed from the base of the PR and between 634e4b5 and 64e45f6.

📒 Files selected for processing (1)

• .envrc

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch fix/make_envrc_compatible_with_nixdirenv

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