Automatically label oversized PRs to keep reviews small

[hanayik]

Problem

Large PRs are slow and lead to shallow reviews. We want a nudge toward smaller,
targeted PRs without blocking anyone.

Proposal

A GitHub Actions workflow that measures each PR and applies an
Oversized PR label when it is over budget.

• Budget: 250 changed lines (additions + deletions)
• Counts only .ts, .tsx, .js, .jsx (and .mts, .cts, .mjs, .cjs)
• Skips brand-new files, so a new test or module is free
• Skips the auto-generated asset index files
• Adds a sticky comment listing the largest files
• Removes the label and comment once the PR shrinks under budget
• Advisory only: the check never fails, so nothing is blocked

How it works

• .github/workflows/pr-size.yml runs on pull_request_target so fork PRs can be labeled
• .github/scripts/pr-size.ts does the work through the GitHub REST API
• The PR's code is never checked out or executed
• Dry run locally: bun .github/scripts/pr-size.ts <pr-number> --dry-run

Reference

Against recent merged PRs, #194 came in at 134 lines and #195 at 373 lines
in existing files, so #195 would have been labeled.

Open questions

• Is 250 the right budget?
• Should test files be excluded from the count as well?

[hanayik]

Idea from Niivue meeting:

• put these rules in AGENTS.md (without the explicit PR rule running separately first)
• Could think about pre-commit hooks. We tried this a bit in the past, could revive it.