using james approach of setting it on the indentify call

Closes MAR-172.

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

src/platform/telemetry/providers/cloud/PostHogTelemetryProvider.test.ts

@@ -137,7 +137,13 @@ describe('PostHogTelemetryProvider', () => {
       const callback = hoisted.mockOnUserResolved.mock.calls[0][0]
       callback({ id: 'user-123' })
 
-      expect(hoisted.mockIdentify).toHaveBeenCalledWith('user-123')
+      expect(hoisted.mockIdentify).toHaveBeenCalledWith(
+        'user-123',
+        undefined,
+        expect.objectContaining({
+          first_auth_at: expect.any(String)
+        })
+      )
     })
   })
 

src/platform/telemetry/providers/cloud/PostHogTelemetryProvider.ts

@@ -121,7 +121,9 @@ export class PostHogTelemetryProvider implements TelemetryProvider {
 
             useCurrentUser().onUserResolved((user) => {
               if (this.posthog && user.id) {
-                this.posthog.identify(user.id)
+                this.posthog.identify(user.id, undefined, {
+                  first_auth_at: new Date().toISOString()
+                })
                 this.setSubscriptionProperties()
               }
             })