feat: prepare releases on release branches

[imantsk]

Renames the release preparation flow from tag branches to release branches, adds an optional AI changelog toggle, and dispatches the core-to-pro sync when a release is created. Companion to codesnippetspro/.github-private#1.

Changes

• prepare-tag.yml renamed to prepare-release.yml ("(Release): Prepare"). New auto_changelog boolean input, default off: the release branch, version bump and pull request are always prepared; AI changelog and readme generation only runs when the box is ticked. Run monitoring now matches the dispatched run by name instead of taking the most recent run, so concurrent dispatches from the other repository are never confused.
• create-tag.yml matches release/v* head branches. The new branch scheme is release/v{version}/{edition}, which satisfies the "Enforce branch suffixes" ruleset — the old tag/v* names have been blocked by it since the ruleset was created.
• release.yml gains a sync-pro job that dispatches sync-core-to-pro.yml when a release is created. Guarded to the core repository, so the job is inert once synced into pro. Stable tags open a staged pull request into pro, beta tags into pro-beta; conflicts are AI-resolved where possible and listed for review, and the pull request is never merged automatically.

Rollout

1. Merge this pull request into core.
2. Run the downstream sync so core-beta, pro and pro-beta receive the workflows (dispatch sync-code-snippets.yml with caller codesnippetspro/code-snippets / core).
3. Merge codesnippetspro/.github-private#1, which switches the shared workflow to the new branch names. No release should be prepared between steps 1 and 3.