Add Home Assistant development environment to flake

- Add home-assistant and mosquitto to dev shell
- Create minimal dev-config/ for HA development instance
- Symlink custom integration for easy testing
- Add HA dev files to .gitignore
- No HA codebase committed, just references via Nix

Usage:
  nix develop
  hass --config ./dev-config

Features:
- Isolated from production HA instance
- MQTT broker for testing
- Debug logging enabled
- In-memory database for fast iteration

.gitignore

@@ -28,3 +28,9 @@ system_stats_daemon-*.tar
 # Nix result symlinks
 result
 result-*
+
+# Home Assistant development files
+dev-config/.storage/
+dev-config/home-assistant.log*
+dev-config/deps/
+dev-config/.cloud/