Prepare DocRefract v0.2.4 release

[jinyounghub]

Summary

• bump the CLI, Action, documentation, examples, and release assets to 0.2.4
• publish the package icon and the post-launch activation improvements from PR Improve post-launch activation and document CI onboarding #10 in the next immutable package
• expand NuGet discovery terms for PDF/DOCX/Word comparison searches
• expand the Marketplace Action description for document regression testing searches
• keep the internal consumer proof pinned to the latest already-published v0.2.3 until publication

Validation

• version synchronization check passed at 0.2.4
• locked restore passed with .NET SDK 10.0.302
• Release build: 0 warnings, 0 errors
• tests: 29 passed
• package normalized and local installed-tool smoke passed for version/report metadata and exit codes 0/1/2
• nupkg contains docrefract-icon.png, the updated description, and all required search tags
• changed YAML files parse successfully
• independent release audit/review found no P0/P1 blocker

No community posts or third-party outreach are included.