Track TypeScript errors in CI

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.

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