[extension] Update DocGuard — CDD Enforcement extension to v0.31.0

[github-actions]

Update the docguard community extension catalog entry from v0.30.0 to v0.31.0.

Changes

• extensions/catalog.community.json: Updated version → 0.31.0, download_url → new release asset, updated_at → 2026-07-08T00:00:00Z. Preserved created_at, downloads, and stars.
• docs/community/extensions.md: No row changes needed (name, description, category, and effect are unchanged).

Validation Results

All checks passed ✅

Check	Result
Extension ID format (^[a-z][a-z0-9-]*$)	✅ docguard
Version follows semver	✅ 0.31.0
Repository exists and is public	✅ https://github.com/raccioly/docguard
extension.yml present	✅ extensions/spec-kit-docguard/extension.yml
README.md present	✅ extensions/spec-kit-docguard/README.md
LICENSE file present	✅ extensions/spec-kit-docguard/LICENSE
GitHub release v0.31.0 exists	✅ DocGuard v0.31.0
Download URL matches release asset	✅ spec-kit-docguard-v0.31.0.zip
Testing checklist fully checked	✅ All 5 items
Submission requirements fully checked	✅ All 6 items

Closes #3401
cc @raccioly

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