Track TypeScript errors in CI #8786

CI runs lint and unit tests but not tsc. Main has about 890 type errors across 351 files, and nothing tells us when a PR adds more.

Add a report on every PR that compares its type errors with main's, without blocking the merge.

• On push to main: run tsc --noEmit, upload per-file error counts as an artifact
• On PRs: run tsc once, compare with main's counts for the merge base (fall back to the latest main)
• Sticky PR comment, like the Visual Regression one:
  • 🟢 "Type errors: 890 → 884 (−6)" when the PR fixes some
  • 🔴 "Type errors: 890 → 892 (+2)", listing the files that gained errors
  • nothing to report when unchanged
• Not a required check, so it never blocks a merge

Later, once it has proved quiet: consider making it required.

The biggest single file is common/stores/feature-list-store.ts (147 errors), which shrinks with the Flux migration.