Support rebase merging for single-commit release PRs

[samuel-williams-shopify]

The generated review rules currently block rebase merging even when a repository enables it. Allow merge, squash, and rebase in those rules, leaving the available methods to repository settings.

Add gem:github:release:validate and use it during preparation and PR validation. It regenerates release content through bake-gem and requires exactly one commit when the version changes. Ordinary PRs retain unrestricted commit counts. Publishing continues to verify the exact resulting commit against its first parent.

Update the generated workflow, rules, guides, agent context, and release notes. Existing installations should regenerate and merge the validation workflow before applying the updated rules; release corrections should amend or regenerate the single release commit.

Validation:

• Full test suite with standard Covered integration: 539/539 library lines covered, plus the final consolidated merge tests.
• Real Git coverage for squash, rebase, and merge commits, including later development on the default branch.
• Rejection of multi-commit releases and unrelated release content; ordinary multi-commit PRs accepted.
• RuboCop and documentation coverage.
• Repeated setup generation produces no further changes.