fix: repair dead links in README and add link-check CI (closes #7910 #7908 #7886 #7885 #7792)

[kang666-kang]

Summary

• Replaced broken CI badge (ci.yml → link-check.yml) — fixes Bug: Broken documentation links in README.md #7886
• Fixed dead /api/tokenomics link → local tokenomics doc — fixes Bug: Broken API endpoint link in documentation - /api/tokenomics returns 404 #7910, [BUG] README.md links to /bcos/ and /api/tokenomics return 404 #7908
• Verified BCOS links already resolve correctly (BCOS.md)
• Verified mining/attestation links ([BUG] README.md has broken link to mining documentation #7885, [Bug] Broken relative documentation links in main README.md #7792) no longer in README
• Added lychee link-check CI to prevent future regressions

Test

• Link-check CI workflow will run on this PR
• All relative links verified against repository contents

[github-actions]

Welcome to RustChain! Thanks for your first pull request.

Before we review, please make sure:

• Non-doc PRs have a BCOS-L1 or BCOS-L2 label
• Doc-only PRs are exempt from BCOS tier labels when they only touch docs/**, *.md, or common image/PDF files
• New code files include an SPDX license header
• You've tested your changes against the live node

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[kang666-kang]

I'm working on this. PR: #8040