feat: Event setup mode

[daniloc]

This pull adds experimental event setup for Nexts.js 15.3+:

src/nextjs/event-setup.ts

This adds the new workflow, including a very thorough prompt with rules that prevent stupid changes from happening. In addition, the workflow creates a markdown report at the end explaining what happened and providing next steps.

Other changes

I needed debug logging for query.ts at various points, so that's in here too. Can take or leave.

Testing

I found a nice and meaty project to test this on:

https://github.com/nextjs/saas-starter

[daniloc]

a couple remaining to-dos:

• Check state of git and warn if uncommitted files
• Analytics events so I can get emails from people who try this

[daniloc]

@jonathanlab welp: it's not nearly as ambitious as I'd hoped but: constraining to client-side files at least provides consistent, reliable outcomes. It gets the ball rolling, and when we have a proper library for this framework we can take another pass for the more compelling version!

[jonathanlab]

This is genuinely so nice: I think it's a great start to just try this out with client side analytics first. Excited to see how people use it!

I ran into an issue with the next-js-instrumentation-client-app where it would sometimes fail to enhance one of the pages available:

│  Enhancing files with event tracking. Changes will be applied as they come in. Use your git interface to review new events. Feel free to toss anything you don't like...
│
◇  ✗ Failed to enhance src/app/example/page.tsx
│
◇  ✓ Enhanced src/app/page.tsx with 3 events
│
└  Success! Added 3 events across 1 files.
    
    Event tracking plan saved to: event-tracking-report.md
    
    Next steps:
    1. Review changes with your favorite git tool
    2. Revert unwanted changes with git checkout <file>
    3. Test that events are being captured in your PostHog project
    4. Create insights in PostHog

I ran it again and it enhanced the failed file successfully, so more of a transient error I guess 🤷🏻