docs(claude): codify friendly New Features / Fixes changelog format

Update the 'Writing changelog entries' guidance to match the rewritten CHANGELOG:
friendly New Features / Fixes sections (Breaking Changes / Security only when
present), one plain sentence per bullet, strip internal paths/symbols/benchmarks,
keep #PR refs + contributor thanks. Notes why multi-word headings are safe on the
normal release path.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>