ci: catch broken markdown links

[x-n2o]

Proposed changes

Adds a markdown link check to the existing pre-commit workflow so repository-local links are validated locally and in pull request CI. Per review feedback, this now uses a small local script (tools/check_links.py) instead of a third-party dependency — the same checker proposed in ml-explore/mlx-swift-examples#479; the two copies are kept in sync.

• pre-commit passes the tracked markdown files to the script; links with a URL scheme or host are skipped — matching the previous offline lychee behavior without the extra dependency. The script also supports a manual whole-tree scan: python3 tools/check_links.py .
• The earlier .lycheeignore is replaced by a .check-links-ignore blocklist (fnmatch globs, repo-root relative). Patterns skip both scanned files and link targets, covering the review feedback:
  • Third-party sources we don't build docs for are not scanned at all: Source/Cmlx/fmt/*, Source/Cmlx/json/*, Source/Cmlx/metal-cpp/* (this also silences false positives like C++ lambdas [=](auto out) in fmt's docs being parsed as markdown links).
  • DocC cross-doc-package links (mlx, mlxnn, mlxoptimizers) are not flagged — they resolve in the built documentation, not as repository files.
• Links inside fenced code blocks are ignored, so the linkage example in MAINTENANCE.md is not flagged.

Unlike the examples repo (#479), no genuinely missing in-repo .md targets turned up: running the checker with an empty .check-links-ignore flags exactly the 7 links covered above — the fmt lambda and the six DocC cross-package links — all verified false positives, so there is nothing to track in issues here.

Checklist

Put an x in the boxes that apply.

• I have read the CONTRIBUTING document
• I have run pre-commit run --all-files to format my code / installed pre-commit prior to committing changes
• I have added tests that prove my fix is effective or that my feature works
• I have updated the necessary documentation (if needed)

Local run:

❯ pre-commit run --all
swift-format.............................................................Passed
check markdown links.....................................................Passed
cmake-format.............................................................Passed

False-positive audit — with an empty .check-links-ignore, the whole-tree scan flags only the known false positives, no real missing files:

❯ python3 tools/check_links.py .

Source/Cmlx/fmt/doc/api.md:
  line 173: [=](auto out) -> NOT FOUND

Source/MLX/Documentation.docc/MLX.md:
  line 69: [MLXNN](mlxnn) -> NOT FOUND
  line 70: [MLXOptimizers](mlxoptimizers) -> NOT FOUND

Source/MLXNN/Documentation.docc/MLXNN.md:
  line 46: [MLX](mlx) -> NOT FOUND
  line 47: [MLXOptimizers](mlxoptimizers) -> NOT FOUND

Source/MLXOptimizers/Documentation.docc/MLXOptimizers.md:
  line 34: [MLX](mlx) -> NOT FOUND
  line 35: [MLXNN](mlxnn) -> NOT FOUND

Total: 7 broken link(s) in 4 file(s)

[Copilot]

Pull request overview

This PR adds an offline broken-Markdown-link check to the existing pre-commit-driven lint workflow (both locally and in CI) by introducing the lychee pre-commit hook, and updates the lint step wording to reflect the broader checks.

Changes:

• Add lychee as a pre-commit hook (offline) to detect broken links in Markdown files.
• Update the CI lint step name/message to mention Markdown link checks.
• Add an acknowledgment entry for the contributor who introduced the link checks.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 2 comments.

File	Description
ACKNOWLEDGMENTS.md	Adds contributor acknowledgment for the new Markdown link checks.
.pre-commit-config.yaml	Introduces the lychee hook configured for offline Markdown link checking.
.github/workflows/pull_request.yml	Renames the lint step and updates the failure message to include Markdown link checks.

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[davidkoski]

This looks interesting, but:

This is a third party and we don't build docs for this -- we shouldn't be checking this.

[Source/Cmlx/fmt/doc/index.md]:
 [ERROR] file:///Users/ledio/src/mlx-swift/Source/Cmlx/fmt/doc/api#compile-api (at 60:12) | File not found. Check if file exists and path is correct
 [ERROR] file:///Users/ledio/src/mlx-swift/Source/Cmlx/fmt/doc/api#compile-time-checks (at 27:10) | File not found. Check if file exists and path is correct
 [ERROR] file:///Users/ledio/src/mlx-swift/Source/Cmlx/fmt/doc/api#format (at 23:52) | File not found. Check if file exists and path is correct
 [ERROR] file:///Users/ledio/src/mlx-swift/Source/Cmlx/fmt/doc/api#udt (at 46:10) | File not found. Check if file exists and path is correct

These links do resolve -- they are cross doc package links, see: https://swiftpackageindex.com/ml-explore/mlx-swift/main/documentation/mlx

[Source/MLX/Documentation.docc/MLX.md]:
 [ERROR] file:///Users/ledio/src/mlx-swift/Source/MLX/Documentation.docc/mlxnn (at 69:3) | File not found. Check if file exists and path is correct
 [ERROR] file:///Users/ledio/src/mlx-swift/Source/MLX/Documentation.docc/mlxoptimizers (at 70:3) | File not found. Check if file exists and path is correct

[Source/MLXNN/Documentation.docc/MLXNN.md]:
 [ERROR] file:///Users/ledio/src/mlx-swift/Source/MLXNN/Documentation.docc/mlx (at 46:3) | File not found. Check if file exists and path is correct
 [ERROR] file:///Users/ledio/src/mlx-swift/Source/MLXNN/Documentation.docc/mlxoptimizers (at 47:3) | File not found. Check if file exists and path is correct

[Source/MLXOptimizers/Documentation.docc/MLXOptimizers.md]:
 [ERROR] file:///Users/ledio/src/mlx-swift/Source/MLXOptimizers/Documentation.docc/mlx (at 34:3) | File not found. Check if file exists and path is correct
 [ERROR] file:///Users/ledio/src/mlx-swift/Source/MLXOptimizers/Documentation.docc/mlxnn (at 35:3) | File not found. Check if file exists and path is correct

[x-n2o]

Ah, I suppose this would require a gating mechanism or a mutli-repo checkout (overengineering?). I found .lycheeignore. I'll add the third-parties that resolve on swiftpackageindex.com/ml-explore.