ci: add link-check workflow (lychee)

[satyakwok]

Same pattern as awesome-sentrix's link-check workflow. Lychee scans every markdown link on PR + push to main + a weekly Monday 03:00 UTC cron. Non-blocking, mirroring gitleaks policy.

Summary by CodeRabbit

• Chores
  • Implemented automated link validation for documentation to detect and prevent broken references.

[coderabbitai]

Caution

Review failed

Pull request was closed or merged during review

📝 Walkthrough

Walkthrough

This PR adds a new GitHub Actions workflow file to automate checking for dead links in markdown documentation. The link-check workflow runs on pull requests, pushes to the main branch, a weekly Monday schedule, and manual trigger. It scans all **/*.md files using lycheeverse/lychee-action, excludes mail-related links, configures concurrency limits, and returns non-blocking output so link findings do not fail the workflow.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Possibly related PRs

• Sentriscloud/dapp-starter#13: Modifies GitHub Actions workflows to pin third-party action references to specific commit SHAs for supply chain security.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Description check	⚠️ Warning	The description is incomplete. It lacks the required 'Test plan' section with a checklist, and the 'Related' section for linking issues or PRs is missing.	Add a 'Test plan' section with a checklist of test scenarios and a 'Related' section to link any relevant issues or pull requests.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'ci: add link-check workflow (lychee)' clearly and concisely summarizes the primary change: adding a new CI workflow for link checking using the lychee tool.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

_{✏️ Tip: You can configure your own custom pre-merge checks in the settings.}

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch ci/link-check

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