feat(onboarding): add tracking for Vercel GitHub steps

Add telemetry events to the Vercel onboarding modal to better track
user interactions with the GitHub integration flow. Specifically:
- Fire an event when the GitHub step is shown, including whether the
  GitHub app is already installed (github_app_installed).
- Track when the user clicks the "Install GitHub app" button.
- Track when the user completes the GitHub onboarding step.
- Track when the user skips the GitHub onboarding step (applies to
  multiple skip paths).

Also add gitHubAppInstallations.length to the effect dependency array
to ensure the "step viewed" event updates correctly when installation
state changes.

These events improve analytics for onboarding funnels and help identify
drop-off or friction points in the GitHub integration flow.

Author: 0ski

apps/webapp/app/components/integrations/VercelOnboardingModal.tsx

@@ -543,8 +543,11 @@ export function VercelOnboardingModal({
 
     if (!isGitHubConnectedForOnboarding) {
       setState("github-connection");
+      trackOnboarding("vercel onboarding github step viewed", {
+        github_app_installed: gitHubAppInstallations.length > 0,
+      });
     }
-  }, [vercelStagingEnvironment, pullEnvVarsBeforeBuild, atomicBuilds, discoverEnvVars, syncEnvVarsMapping, nextUrl, fromMarketplaceContext, isGitHubConnectedForOnboarding, completeOnboardingFetcher, actionUrl, trackOnboarding]);
+  }, [vercelStagingEnvironment, pullEnvVarsBeforeBuild, atomicBuilds, discoverEnvVars, syncEnvVarsMapping, nextUrl, fromMarketplaceContext, isGitHubConnectedForOnboarding, completeOnboardingFetcher, actionUrl, trackOnboarding, gitHubAppInstallations.length]);
 
   const handleFinishOnboarding = useCallback((e: React.FormEvent<HTMLFormElement>) => {
     e.preventDefault();
@@ -1081,6 +1084,7 @@ export function VercelOnboardingModal({
                       )}
                       variant="secondary/medium"
                       LeadingIcon={OctoKitty}
+                      onClick={() => trackOnboarding("vercel onboarding github app install clicked")}
                     >
                       Install GitHub app
                     </LinkButton>
@@ -1110,6 +1114,7 @@ export function VercelOnboardingModal({
                     <Button
                       variant="primary/medium"
                       onClick={() => {
+                        trackOnboarding("vercel onboarding github completed");
                         setState("completed");
                         const validUrl = safeRedirectUrl(nextUrl);
                         if (validUrl) {
@@ -1123,6 +1128,7 @@ export function VercelOnboardingModal({
                     <Button
                       variant="tertiary/medium"
                       onClick={() => {
+                        trackOnboarding("vercel onboarding github skipped");
                         setState("completed");
                         if (fromMarketplaceContext && nextUrl) {
                           const validUrl = safeRedirectUrl(nextUrl);
@@ -1141,6 +1147,7 @@ export function VercelOnboardingModal({
                     <Button
                       variant="tertiary/medium"
                       onClick={() => {
+                        trackOnboarding("vercel onboarding github skipped");
                         setState("completed");
                       }}
                     >