docs(claude-md): credit each changelog entry to its pull request and author

[datlechin]

Release notes are built from CHANGELOG.md, and they credited nobody. GitHub's own generated notes end every line with by @author in #123. This gives every released entry the same credit, in the parens it already ends with:

- Nested Elasticsearch fields showing as null in the grid, and filters on those leaves matching nothing. (#2905 by @digows)
- OceanBase MySQL-mode connection type on the MySQL driver. (#1748, #2741 by @J2TeamNNL)

Why the release adds it, not the pull request

A pull request has no number until it is opened, so an entry written with the change cannot carry one, and a contributor should not need a second commit to add it. scripts/ci/changelog_credits.py stamps it at release time from the history:

1. git blame names the commit that wrote each entry line.
2. The squash merge subject ends with the pull request number.
3. gh pr view names that pull request's author, the person credited, not whoever merged it.

It folds the credit into an existing issue reference ((#1748) becomes (#1748, #2741 by @J2TeamNNL)), never repeats a number that is already there, and never touches an entry that is already credited. An entry whose commit has no pull request number, or that is not committed yet, is left bare and listed so the release can credit it by hand.

Dry run against [Unreleased] as it stands: 236 entries credited across 81 pull requests, none left bare. The three contributors' entries (@J2TeamNNL #2741, @digows #2905, @filipac #2927) land on their own lines. Blame credits the last commit to touch a line, so a maintainer rewording a contributor's entry later would take the credit; each contributor PR's lines were checked against their current blame and none had been.

What changed

• scripts/ci/changelog_credits.py and test_changelog_credits.py (15 cases), run in Repo Hygiene next to the release note tests.
• CLAUDE.md rule 1 states the credit and that the release, not the pull request, adds it.
• The release skill runs it as pre-flight step 6, before any rewording. A line reworded in the working tree blames to nothing and would be skipped. The length check now measures entries without the credit.
• CONTRIBUTING.md tells contributors to leave the credit off.