Fix build workflow failure caused by dead internal project links

[Copilot]

The build GitHub Actions job failed in htmlproofer because the generated project tab contained internal links to non-existent /projects/* pages. This PR removes those dead links while preserving the placeholder project entries.

• Failure source

  • _tabs/project.md had placeholder items inside an HTML comment block, but those lines still used Markdown link syntax targeting routes that do not exist.

• Change applied

  • Replaced commented placeholder Markdown links with plain text entries (no /projects/* href targets).
  • Kept all placeholder content and intent intact; only link markup was removed.

• Resulting behavior

  • The project tab still shows/retains placeholder content context, but no longer introduces invalid internal links into the generated site.

• Before/after snippet

  <!-- before -->
  - [数字时钟](/projects/digital-clock/) — C++ + SFML 桌面时钟程序
  
  <!-- after -->
  - 数字时钟 — C++ + SFML 桌面时钟程序

[Copilot]

Pull request overview

This PR addresses a GitHub Actions build failure caused by htmlproofer detecting dead internal links generated from placeholder project entries in the Projects tab.

Changes:

• Removed Markdown link markup from placeholder project entries that pointed to non-existent /projects/* pages.
• Preserved the placeholder entries’ text and intent while eliminating invalid internal link targets.

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