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: 8be628f6-39ed-4916-addd-86e1ad8fa785

📥 Commits

Reviewing files that changed from the base of the PR and between 37a56c8 and 3907d12.

📒 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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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 28.71%. Comparing base (37a56c8) to head (3907d12).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #565   +/-   ##
=======================================
  Coverage   28.71%   28.71%           
=======================================
  Files         175      175           
  Lines        7059     7059           
=======================================
  Hits         2027     2027           
  Misses       4913     4913           
  Partials      119      119           

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