🤖 chore: migrate .envrc to nix-direnv bootstrap

Switch .envrc from plain 'use flake' to nix-direnv bootstrap with pinned version/hash, then keep use flake.

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_Generated with `mux` • Model: `openai:gpt-5.3-codex` • Thinking: `xhigh` • Cost: `$0.11`_

<!-- mux-attribution: model=openai:gpt-5.3-codex thinking=xhigh costs=0.11 -->

Author: ThomasK33

.envrc

@@ -1 +1,4 @@
+if ! has nix_direnv_version || ! nix_direnv_version 3.1.1; then
+  source_url "https://raw.githubusercontent.com/nix-community/nix-direnv/3.1.1/direnvrc" "sha256-p+fzQdrms/hDa7g+soShAybJNo4bN4SIAeSfqNKgD5I="
+fi
 use flake