Use uniform release headers in CHANGES.md

[fingolfin]

This renames CHANGES to CHANGES.md and starts each release entry with a uniform header, ## VERSION (YYYY-MM-DD). We are moving the changelogs of the packages in gap-packages to this format, so that scripts can extract the notes of each release.

Dates missing from the file, or given only as a month, are taken from the git history: the date in PackageInfo.g at the release tag, or the date of the tagged commit. Where neither exists, the entry keeps its month (YYYY-MM) or has no date. Otherwise only headers and formatting change: indentation, blank lines, separator lines and list markers.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 54.52%. Comparing base (41943e0) to head (fd4f781).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #155   +/-   ##
=======================================
  Coverage   54.52%   54.52%           
=======================================
  Files         109      109           
  Lines       40334    40334           
  Branches     2353     2353           
=======================================
  Hits        21993    21993           
  Misses      18043    18043           
  Partials      298      298           

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.