Add docs-drift guard to CI

As mechababs grows, it'll be a chore to keep the docs up-to-date, and even if the CLAUDE.md is explicit, "always keep the docs up to date" still seems to fail for me (across repos), especially in long-running sessions with large context, and even more so when I say "skip for now we'll come back to it". An explicit secondary check in the CI seems helpful.

Built a very alpha version, which will probably need changes before useful here: https://github.com/asmacdo/pr-scout
Theres probaby better versions elsewhere, or maybe even just enabling copilot autoreview with explicit instructions about docs drift may perform better.

Experimentally used a tiny bit in yolo.
https://github.com/con/yolo/blob/main/.github/workflows/pr-scout.yml

Start by reviewing the alpha pr-scout project and the example .github/workflows/pr-scout.yml from yolo, then compare them with mechababs' current documentation workflow and CLAUDE.md guidance. Decide on the CI approach before implementation; done means an agreed secondary check is integrated and reliably reports documentation drift.

Written by the indexing model from the issue text.