chore(release): check_release.sh, a beta preparation helper and version-bump for pre-releases #1498

Part of the Beta channel epic #1499.

Problem

Cutting a beta by hand needs five edits that must agree (pubspec, CHANGELOG section, tag, release branch name, AUR / metainfo versions) and the failure shows up late, in the Publish GitHub Release job. Nothing checks it before the tag is pushed.

Scope

• scripts/release/check_release.sh <tag>: fails unless pubspec semver equals the tag (without v), the CHANGELOG has ## [<tag>], the tag is not already used, and, for a tag with -, the pubspec build number is present. Prints what the workflow will do (pre-release or stable, which package jobs run).
• scripts/release/prepare_beta.sh X.Y.Z N (optional): sets version: X.Y.Z-beta.N+B, adds an empty ## [X.Y.Z-beta.N] section from [Unreleased], and prints the tag command. Never commits or pushes.
• version-bump.yml: when the PR is a release branch PR carrying a pre-release version, do not bump (or document why it is safe); today it strips -beta.N on merge to main.
• A job in ci.yml that runs check_release.sh on PRs from release/* (dry mode).
• version-hint reuses the same checks.

Acceptance

• check_release.sh 0.5.0-beta.1 on a prepared release branch passes; on a missing CHANGELOG section it names the missing section.
• A PR from release/* runs the check in CI.
• A beta version in pubspec survives the merge of its release PR (or the doc explains the manual step precisely).