Changelog skill misses version.txt update #7508
Description
The changelog-generation skill delegates file update instructions to a reference file (scope-rules.md) instead of listing them inline. The Copilot coding agent doesn't always follow the reference, so version.txt gets skipped.
Previous agents had the file list inline and never missed it (e.g. #7350 worked, #7500 missed it).
Regression from #7381 (skill migration).
Fix: list the files to update inline in SKILL.md Step 2, keep the reference for version derivation details.