docs: make local LibreOffice round-trip tests a required step for ODT changes and releases

[vaceslav]

Closes #218 (decision: no LibreOffice CI job; the tests are a required local step instead).

• CLAUDE.md: new section "LibreOffice Round-Trip Tests (mandatory, run locally)" covering when to run them (ODT/shared engine changes and every release), the command, and that 0 skipped is required.
• CONTRIBUTING.md:
  • the same rule under Testing Requirements;
  • the release checklist now includes the LibreOffice run, updating the release PR branch so CI runs, and the post-release PublicAPI/baseline step;
  • the "don't edit CHANGELOG by hand" rule now describes the release-time upgrade-notes exception we actually use.

Verified locally: dotnet test ... -f net10.0 --filter "Category=LibreOffice" → 15/15 passed, 0 skipped.

Public API impact: none.

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

✅ All modified and coverable lines are covered by tests.

📢 Thoughts on this report? Let us know!