feat: add broken-link drift checker

[advancedresearcharray]

Summary

Adds a broken-link drift checker for #52:

• Scans scaffold markdown for [text](path) links with local/relative targets
• Flags targets that do not exist (resolved relative to the containing file, then project/scaffold roots)
• Skips http(s)://, mailto:, #anchors, and links inside fenced or inline code

Test plan

• npm run typecheck && npm test && npm run build
• Unit tests: broken link, valid link, external/anchor ignored, code-span ignored

Closes #52

[theDakshJaitly]

Thanks for another checker, @advancedresearcharray — this one's noticeably more ambitious than #60, and it shows. The fenced-code and inline-code stripping is genuinely thoughtful (and you tested both), the file-relative + .mex/-prefix resolution mirrors path.ts correctly, and catching image links (![alt](path)) as a bonus is a nice touch. Tests pass and the structure is clean.

I'm marking this request-changes for two things before merge — both straightforward:

1. Local links with a #fragment are falsely flagged as broken (see inline). A link like [install](./target.md#install) reports BROKEN_LINK even when target.md exists, because the #install fragment is never stripped before the existence check. I reproduced it against the checker. Linking to a heading in another doc is a common pattern, and at error severity (-10 each) those false positives add up. The fix is small — strip a trailing #... (and ideally ?...) in normalizeLinkTarget.

2. Rebase on main and recount the checkers. #60 just merged and touched the same README / CONTRIBUTING / CHANGELOG lines plus index.ts / reporter.ts / types.ts / checkers.test.ts, so this branch now conflicts on 6 files. After rebasing, main already lists ten checkers (it added tool-config-sync + todo-fixme), so with broken-link the count becomes eleven — the intro line should read "Eleven," and the table just needs the broken-link row added (the rest are already there). The CONTRIBUTING "Adding a drift checker" count moves to 11 too. (Tiny thing while you're in CHANGELOG: main uses ## [Unreleased] with brackets to match the ## [0.3.5] style — yours dropped the brackets.)

Non-blocking, optional follow-ups:

• Severity: path.ts downgrades to warning for patterns/ files and placeholder paths, reserving error for real misses — worth considering the same here so a pattern doc demonstrating link syntax doesn't hard-error.
• Reference-style links ([a][ref]), paths with spaces, ~~~ fences, and double-backtick inline code aren't handled — all fine to leave; a one-line scope comment would be plenty.

Really solid work overall. Once the fragment fix and the rebase/recount are in, this is good to go. Thanks again for taking on the harder checkers.

[theDakshJaitly]

This is where the #fragment false positive originates. For [install](./target.md#install), target stays ./target.md#install, which fails existsSync, so an existing file is reported broken (at error severity). Reproduced locally.

Suggested fix — drop the fragment/query before returning:

function normalizeLinkTarget(raw: string): string {
  let target = raw.replace(/^<|>$/g, "").trim();
  const titleSplit = target.match(/^([^\s]+)(?:\s+["'].+["'])?$/);
  if (titleSplit) target = titleSplit[1];
  // Drop in-page fragment / query so links to a heading in another file resolve
  target = target.replace(/[#?].*$/, "");
  return target;
}

Bonus: a pure #section link then normalizes to "" and is already skipped by the !target guard up in the loop, so same-page anchors stay ignored without needing the startsWith("#") branch. Worth a test for [x](./target.md#heading) (target exists) -> expect no issue.

[theDakshJaitly]

Optional / non-blocking: path.ts (the closest analog) emits warning rather than error for patterns/ files and placeholder-y paths, keeping error for genuinely-missing real paths. Since a broken link is the same class of problem, mirroring that would stop example links in pattern docs from costing 10 points each.

[advancedresearcharray]

Pushed fixes for the review in 73570d8: strip #/? from link targets before existsSync, add test for ./target.md#install, and downgrade broken links under patterns/ to warning (matching path.ts). Ready for another look when you have a moment.

[advancedresearcharray]

Rebased onto main (post-#60) and resolved conflicts. Branch now includes both todo-fixme and broken-link; README/CONTRIBUTING/CHANGELOG updated to 11 checkers. Fragment stripping + patterns/ warning severity unchanged in d4d3d9b. All 175 tests pass locally.

[theDakshJaitly]

Re-reviewed after the rebase — all good now.

• Fragment fix is in (normalizeLinkTarget strips [#?].*$). I re-checked independently: ./target.md#install and query strings resolve, and a genuinely-missing file with a fragment still flags, so the strip isn't over-broad. Good regression test for it too.
• Rebased cleanly on main; checker count reconciled to eleven across the README intro, the table, and CONTRIBUTING, and the CHANGELOG [Unreleased] bracket style is preserved.
• You also picked up the optional severity nuance (warning in patterns/, mirroring path.ts) with a test — appreciated.

typecheck / 175 tests / build all green locally. Approving.

[theDakshJaitly]

Merged — and this is your first contribution to land in mex. Thank you for taking on two of the trickier good-first-issues back to back, and for the clean turnaround here: the rebase, the fragment fix, and picking up the severity nuance unprompted all made this easy to ship. Genuinely solid work. Hope to see more from you whenever you're up for it.