Preview requests use GitHub App tokens to avoid shared user rate-limit failures

Tuist Previews now authenticates to GitHub using short-lived GitHub App installation tokens instead of a shared bot user’s long-lived token. This fixes preview requests that could fail while posting progress back to Slack because they shared the same user-level GitHub rate limit. The preview workflow boundary remains unchanged, but operators should populate the GitHub App fields in the TUIST_OPS_BOT secret before deploying.