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

dev-config/automations.yaml

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+# Development automations for testing Systant integration
+[]

dev-config/configuration.yaml

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+# Home Assistant Development Configuration for Systant Integration
+
+homeassistant:
+  name: Systant Development
+  latitude: 0
+  longitude: 0
+  elevation: 0
+  unit_system: metric
+  time_zone: UTC
+  
+# Enable configuration UI
+config:
+
+# Enable frontend
+frontend:
+  themes: !include_dir_merge_named themes
+
+# Enable API
+api:
+
+# Enable system health
+system_health:
+
+# Enable mobile app support
+mobile_app:
+
+# MQTT Configuration (connect to local broker)
+mqtt:
+  broker: localhost
+  port: 1883
+  discovery: true
+  discovery_prefix: homeassistant
+
+# Enable logging
+logger:
+  default: info
+  logs:
+    custom_components.systant: debug
+    homeassistant.components.mqtt: debug
+
+# Enable recorder with in-memory database for development
+recorder:
+  db_url: "sqlite:///:memory:"
+  purge_keep_days: 1
+
+# Enable default integrations
+default_config:

dev-config/scenes.yaml

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+# Development scenes for testing Systant integration
+[]

dev-config/scripts.yaml

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+# Development scripts for testing Systant integration
+{}