Fwiw i feel like you could still do both, no? Ie run all tests daily or whatever cadence is not painful but still protective, but _also_ use heuristics to improve PR responsiveness.

Like anything it's about tradeoffs. Though if it were me i'd simply write a mechanism which deterministically decides which areas of code pertain to which tests and use that simple program to determine which tests run. The algorithm could be loosely as simple as things like code owners and git-blame, but relative to some big set of Code->Test that you can have Claude/etc build ahead of time. The difference being it's deterministic between PRs and can be audited by humans for anything obviously poor or w/e.

As much as LLMs are interesting to me i hate using them in places where i want consistency. CI seems terrible for them.. to me at least.