chore(claude): Add changelog updating to PR and standalone skills

[rekmarks]

Summary

Integrates changelog updating into the Claude Code PR creation workflow and adds a standalone /update-changelogs skill. Moves the changelog documentation from the README into a dedicated file at docs/contributing/updating-changelogs.md so it can be referenced by both skills and human contributors.

• Add a mandatory "Update changelogs" phase (Phase 4) to the PR skill, before PR creation
• Add a standalone update-changelogs skill for ad-hoc changelog updates
• Move changelog documentation from README.md to docs/contributing/updating-changelogs.md with minor wording improvements
• Update .gitignore to track docs/contributing/

Testing

These changes are to documentation and Claude Code skill definitions. Verified that:

• The relative links from skill files to the changelog docs resolve correctly
• The .gitignore change correctly tracks the new docs/contributing/ directory
• The README link to the extracted documentation is valid

🤖 Generated with Claude Code

---

Note

Low Risk
Low risk: changes are limited to documentation, Claude skill instructions, and .gitignore rules with no runtime code or release logic changes.

Overview
Adds a mandatory "Update changelogs" phase to the .claude/skills/pr workflow, requiring contributors to follow docs/contributing/updating-changelogs.md and commit changelog updates before running gh pr create.

Introduces a standalone .claude/skills/update-changelogs skill for ad-hoc changelog updates, moves the changelog guidance from README.md into docs/contributing/updating-changelogs.md, and updates .gitignore so docs/contributing/ is tracked.

^{Reviewed by Cursor Bugbot for commit d4c0024. Bugbot is set up for automated code reviews on this repo. Configure here.}