chore: add direnv .envrc for per-repo nix shell (#19)

Auto-loads the repo's own flake devshell on `cd` via direnv. No-op for non-Nix users (direnv simply isn't installed). `.direnv/` eval cache gitignored.

Part of dropping the redundant root migration/ flake aggregator (its non-git path: input copied the whole tree and broke on the local postgres data dir). Each repo's own git-scoped flake is the source of truth now.

Co-authored-by: archvillainette <zoelynne.victoria@gmail.com>
Reviewed-on: #19

.gitignore

@@ -18,3 +18,4 @@ result-*
 *.swp
 .idea/
 .vscode/
+.direnv/