docs: simplify changelog release titles and dates

[yechang15]

Summary

• Add concise feature themes to significant releases; retain date-only titles for smaller updates and fixes.
• Use YYYY-MM-DD dates throughout release headings to improve table-of-contents readability.
• Remove the September 22, 2026 release entry as requested.
• Preserve all other historical release descriptions and keep individual feature descriptions out of the heading hierarchy.

Validation

• Full pnpm run validate passed for the feature-title changes (32 Playwright tests).
• Rebuilt the documentation and passed both changelog heading/layout tests after the date-format and entry-removal updates.
• Visually checked the local changelog and table of contents.
• git diff --check passed.

No release needed: documentation and tests only; no changeset required.