[CI/Packaging] Keep publication-pinned commits reachable after merge

[brandonpayton]

Why

Some packaging releases must keep one exact reviewed pull-request commit reachable after the pull request merges. For example, the ABI 42 Homebrew publisher and bottle catalog are pinned to one exact commit. A squash or rebase merge rewrites that identity, but the current merge-candidate activation protocol rejects merge commits. That would leave us with a bad choice: lose the pinned commit or merge successfully and then fail the canonical package activation.

This adds one narrow, fail-closed merge mode for that situation. It does not change the default: ordinary pull requests still squash, and batched-changes pull requests still rebase.

What changed

• Add a preserve-head-commit history mode to Prepare merge.
• Require its final merge commit to have exactly the prepared base and reviewed pull-request head as its ordered parents.
• Continue requiring the merged tree to equal the tree that Prepare merge tested.
• Reject a pull request that carries both batched-changes and preserve-head-commit before creating a candidate.
• Document the bounded repository-settings window: merge commits may remain disabled except while an approved preserve-head merge is performed.

Validation

• bash scripts/dev-shell.sh bash .github/scripts/test-verify-merge-candidate.sh
• bash scripts/dev-shell.sh bash .github/scripts/test-init-merge-candidate.sh
• bash scripts/dev-shell.sh bash .github/scripts/test-merge-candidate-workflows.sh
• bash scripts/dev-shell.sh ruby -e 'require "yaml"; YAML.parse_file(".github/workflows/prepare-merge.yml")'
• bash -n for every changed shell script
• git diff --check

All listed checks passed. actionlint was not run because it is not declared in the repository dev shell; the repository's merge-candidate workflow contract suite and a YAML parse were run instead.

Scope

This changes CI/package-release history validation only. It does not change the kernel, ABI, runtime behavior, package bytes, or the frozen head of PR #1079.

[github-actions]

prepare-merge: runtime/materialization tests passed against the synthetic PR merge; package staging and durable package publishing were skipped. merge-gate=success posted on PR HEAD. This PR is ready for a reviewer to squash merge.