📝 Prepare changelog and upgrade guide at release time

[simon1hofmann]

🤖 AI text below 🤖

Description

Feature and fix PRs currently edit shared Unreleased sections, causing merge conflicts. Prepare CHANGELOG.md and UPGRADING.md in release-preparation PRs. Contributors describe user-facing changes, limitations, and required migrations in their PR descriptions so release maintainers can collect them.

Update Core's development policy and agent guide, and regenerate the contributor guide and PR checklist from templates #449. Remove both per-PR release-documentation checkboxes and pin the templating workflow to its merge commit 377a3dcd360ed3e4d879901dbda7d21e889f435b. Core’s own policy allows changes to these files only during release preparation or when explicitly requested.

Remove the current changelog's Unreleased entries and both files' Unreleased headers and unused links. Between releases, both files contain published releases only. Release preparation gathers changes and migration instructions from Git history, merged PR descriptions, Release Drafter, and any existing notes, accounting for backports and earlier maintenance releases. The removed changelog notes concern #2537, #2544, and #2547; those PRs remain sources for the next release. The upgrade guide's v4 overview link now targets the published changelog section.

AI tools may draft release documentation; maintainers verify claims, links, attribution, and migration examples. Implements the review discussion in #2550, including preparing the upgrade guide at release time.

Validation:

• uvx nox -s lint passed.
• Regenerated all enabled template consumers from the pinned merge commit using Core’s workflow settings.
• All 13 upstream template rendering tests passed, covering project types with and without changelog and upgrade-guide support.
• Verified that published release sections are preserved and neither release-documentation file retains an Unreleased section or comparison link. The upgrade guide's v4 overview link is corrected.

This is a documentation and template update; no new runtime tests are needed. CI for the updated head remains pending. Codex drafted the changes, regenerated the template output, and ran validation.

Checklist

• The pull request only contains commits that are focused and relevant to this change.
• I have added appropriate tests that cover the new/changed functionality.
• I have updated the documentation to reflect these changes.
• The changes follow the project's style guidelines and introduce no new warnings.
• The changes are fully tested and pass the CI checks.
• I have reviewed my own code changes.

If PR contains AI-assisted content:

• Any agent that created, edited, or submitted GitHub content was explicitly authorized for that scope, as required by our AI Usage Guidelines.
• Every agent-authored or agent-edited public text body begins with the visible disclosure 🤖 *AI text below* 🤖 (titles are exempt).
• I have disclosed AI assistance in the PR description.
• I confirm that I have personally reviewed and understood all AI-generated content, and accept full responsibility for it.

[burgholzer]

Let's also drop the upgrade guide and generate that on the fly in release prep PRs.

[burgholzer]

Just one small correction. Otherwise looks good