ci: migrate unreleased changelog entries to towncrier fragments (#5213)

* migrate unreleased changelog entries to towncrier fragments

Convert 26 existing entries from the ## Unreleased section of
CHANGELOG.md into towncrier fragment files in .changelog/,
completing the migration started in #5187.

The ## Unreleased section is removed since towncrier now manages
changelog generation from fragment files at release time.

Assisted-by: Claude Opus 4.6

* fix fragment PR numbers and categorization

- 5072.fixed → 5077.fixed (5072 was the issue, 5077 is the PR)
- 4907.added → 4908.added (4907 was the issue, 4908 is the PR)
- 5163.changed → 5163.fixed (recategorize as fix)

Per review from @xrmx.

Assisted-by: Claude Opus 4.6