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 in 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.
• AGENTS.md branch naming aligned with the ruleset: edition suffix as the final path segment (/core, /pro) instead of the dash form the ruleset rejects.

Merge order

1. This pull request (and its twin in the other repository, on all release branches).
2. codesnippetspro/.github-private#1, which flips the shared workflow to the new branch names.

[imantsk]

Superseded: #435 into core is the single entry point; the sync flow propagates it to core-beta, pro and pro-beta.