feat: add link-check CI workflow to prevent broken links

[foxxx009]

Summary This PR adds a link-check CI workflow to prevent broken links in documentation. ## Changes - Added `.github/workflows/link-check.yml` with: - Relative link validation: Checks all markdown files for broken relative links (must pass) - External link checking: Uses lychee to check external URLs (non-blocking for transient failures) - Runs on push/PR to main and weekly schedule ## Closes - Closes #7910 - Closes #7908 - Closes #7886 - Closes #7885 - Closes #7792 ## Bounty Bounty: Scottcjn/rustchain-bounties#16248 ## RTC Wallet RTC wallet address will be provided upon merge.

[github-actions]

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[foxxx009]

Done. Added SPDX license header to check_links.py. This is a doc-only PR (CI workflow + link checker script), so BCOS tier labels are not required per the policy.

[Scottcjn]

I ran the relative-link step from this workflow against the repo as it stands. It reports 424 broken links, and the ones I spot-checked all exist. Since that step ends in exit 1, merging this makes CI red on every PR immediately.

The checker has three bugs that account for the false positives:

1. Paths are resolved from the repo root, not from the file containing the link. resolved="$url" never applies the comment above it about handling ./ or ../. So a link from snap/README.md to ../CONTRIBUTING.md is tested as ../CONTRIBUTING.md relative to the repo root and reported broken, when CONTRIBUTING.md is right there.

2. Anchors are not stripped. CONTRIBUTING.md links to SECURITY.md#payment-authority-impersonation, and the script tests for a file literally named SECURITY.md#payment-authority-impersonation.

3. Directory links with a trailing slash. sdk/README.md links to python/, which resolves to sdk/python/ and exists.

Sample output, with the targets confirmed present in the tree:

BROKEN in ./CONTRIBUTING.md: SECURITY.md#payment-authority-impersonation   -> SECURITY.md EXISTS
BROKEN in ./snap/README.md: ../CONTRIBUTING.md                             -> CONTRIBUTING.md EXISTS
BROKEN in ./snap/README.md: ../extension/README.md                         -> extension/README.md EXISTS
BROKEN in ./sdk/README.md: python/                                         -> sdk/python EXISTS
BROKEN in ./wallet/NETWORK_ERROR_HANDLING.md: ../docs/FAQ_TROUBLESHOOTING.md -> docs/FAQ_TROUBLESHOOTING.md EXISTS
TOTAL_ERRORS=424

Two other things:

The external lychee step has both || true and continue-on-error: true, so it can never fail or even mark the job. It is doing nothing that the summary step does not already claim in text.

The repo already has .github/workflows/lychee.yml, which runs lycheeverse/lychee-action@v2 over ./**/*.md and ./**/*.html with --include-fragments and a curated exclude list, and does fail the job on breakage. That workflow already handles relative links, fragments and external links correctly, because lychee resolves paths relative to each input file. A hand-rolled bash and grep checker is not going to beat it, and check_links.py at the repo root is a third copy of the same idea with no tests and a requests dependency.

My recommendation is to close this in favour of the existing lychee.yml. If there is a gap in what lychee currently checks, a small PR adjusting that workflow's arguments would be a much better shape than a parallel checker. Thanks for digging into the link rot either way, it is a real problem in this repo.