Optimize the release workflow

[spawnia]

Motivation

The current release process is entirely manual and discourages frequent releases — see #1891 (comment).

Goals

• Release as soon as changes are merged
• Automate version bumping via semver
• Keep a structured changelog (categorized changes + PR references, similar to current format)
• Keep GitHub releases titled plainly by tag

Proposed approach

Use semantic-release as discussed in #1787, with prior art in https://github.com/mll-lab/php-utils.

1. Enforce conventional PR titles via amannn/action-semantic-pull-request
2. Squash-merge PRs to carry the conventional commit message
3. Run semantic-release on push to master — determines version, updates changelog, creates GitHub release

Open questions

• Should chore: commits (deps, CI) appear in the changelog?
• How to handle migration from the existing CHANGELOG.md?
• Changelog plugin choice — @semantic-release/changelog with custom template, or a community plugin mapping to Keep a Changelog categories?