chore: Add Nix development state to gitignore

Ignore local Nix development directories and files:
- .nix-mix/ - Mix cache
- .nix-hex/ - Hex cache
- .nix-postgres/ - PostgreSQL data directory
- .nix-redis/ - Redis data directory
- .nix-services-started - Service flag file

These are created by the Nix dev shell and should not be
committed to the repository.

.gitignore

@@ -1,6 +1,13 @@
 # Development worktrees
 .worktrees/
 
+# Nix development environment state
+.nix-mix/
+.nix-hex/
+.nix-postgres/
+.nix-redis/
+.nix-services-started
+
 # The directory Mix will write compiled artifacts to.
 /_build/