Add WeatherKit data source package

Implements @aris/data-source-weatherkit for fetching weather data from
Apple WeatherKit REST API.

- WeatherKitDataSource class implementing DataSource interface
- Feed items: current, hourly, daily, and alerts
- Priority adjustment based on weather conditions and alert severity
- Unit conversion (metric/imperial)
- Response validation with arktype
- Test fixtures from real API responses

Co-authored-by: Ona <no-reply@ona.com>

packages/aris-data-source-weatherkit/README.md

@@ -0,0 +1,51 @@
+# @aris/data-source-weatherkit
+
+Weather data source using Apple WeatherKit REST API.
+
+## Usage
+
+```typescript
+import { WeatherKitDataSource, Units } from "@aris/data-source-weatherkit"
+
+const dataSource = new WeatherKitDataSource({
+	credentials: {
+		privateKey: "-----BEGIN PRIVATE KEY-----\n...",
+		keyId: "ABC123",
+		teamId: "DEF456",
+		serviceId: "com.example.weatherkit",
+	},
+	hourlyLimit: 12, // optional, default: 12
+	dailyLimit: 7, // optional, default: 7
+})
+
+const items = await dataSource.query(context, {
+	units: Units.metric, // or Units.imperial
+})
+```
+
+## Feed Items
+
+The data source returns four types of feed items:
+
+| Type              | Description                |
+| ----------------- | -------------------------- |
+| `weather-current` | Current weather conditions |
+| `weather-hourly`  | Hourly forecast            |
+| `weather-daily`   | Daily forecast             |
+| `weather-alert`   | Weather alerts             |
+
+## Priority
+
+Base priorities are adjusted based on weather conditions:
+
+- Severe conditions (tornado, hurricane, blizzard, etc.): +0.3
+- Moderate conditions (thunderstorm, heavy rain, etc.): +0.15
+- Alert severity: extreme=1.0, severe=0.9, moderate=0.75, minor=0.7
+
+## Authentication
+
+WeatherKit requires Apple Developer credentials. Generate a private key in the Apple Developer portal under Certificates, Identifiers & Profiles > Keys.
+
+## Validation
+
+API responses are validated using [arktype](https://arktype.io) schemas.