Add sops-nix for secrets management

- Add flake.nix with sops-nix input
- Configure sops with age key encryption
- Add .sops.yaml template for age key configuration
- Create secrets/ directory for encrypted secrets
- Add .gitignore for age keys and nix result symlinks

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,7 @@
+# Age keys (never commit these)
+*.key
+key.txt
+
+# Result symlinks
+result
+result-*