feat: add Docker/cloud environment detection to telemetry (v2.18.1)

Added isDocker and cloudPlatform fields to session_start telemetry events to enable measurement of the v2.17.1 user ID stability fix.

Changes:
- Added detectCloudPlatform() method to event-tracker.ts
- Updated trackSessionStart() to include isDocker and cloudPlatform
- Added 16 comprehensive unit tests for environment detection
- Tests for all 8 cloud platforms (Railway, Render, Fly, Heroku, AWS, K8s, GCP, Azure)
- Tests for Docker detection, local env, and combined scenarios
- Version bumped to 2.18.1
- Comprehensive CHANGELOG entry

Impact:
- Enables validation of v2.17.1 boot_id-based user ID stability
- Allows segmentation of metrics by environment
- 100% backward compatible - only adds new fields
- All tests passing, TypeScript compilation successful

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

src/telemetry/event-tracker.ts

@@ -175,9 +175,27 @@ export class TelemetryEventTracker {
       platform: process.platform,
       arch: process.arch,
       nodeVersion: process.version,
+      isDocker: process.env.IS_DOCKER === 'true',
+      cloudPlatform: this.detectCloudPlatform(),
     });
   }
 
+  /**
+   * Detect cloud platform from environment variables
+   * Returns platform name or null if not in cloud
+   */
+  private detectCloudPlatform(): string | null {
+    if (process.env.RAILWAY_ENVIRONMENT) return 'railway';
+    if (process.env.RENDER) return 'render';
+    if (process.env.FLY_APP_NAME) return 'fly';
+    if (process.env.HEROKU_APP_NAME) return 'heroku';
+    if (process.env.AWS_EXECUTION_ENV) return 'aws';
+    if (process.env.KUBERNETES_SERVICE_HOST) return 'kubernetes';
+    if (process.env.GOOGLE_CLOUD_PROJECT) return 'gcp';
+    if (process.env.AZURE_FUNCTIONS_ENVIRONMENT) return 'azure';
+    return null;
+  }
+
   /**
    * Track search queries
    */