[codex] Add release stack merge runbook gate

[jzvcpe-goat]

Summary

• add a read-only merge runbook verifier for the stacked release PR chain
• document oldest-to-newest merge flow, draft-to-ready actions, next-PR retargeting to main, and no intermediate branch deletion
• extend the release stack manifest to include this PR and refresh generated evidence bundles

Validation

• python3 -m py_compile scripts/verify_release_stack_readiness.py scripts/verify_release_stack_lineage.py scripts/verify_release_stack_merge_runbook.py
• python3 scripts/verify_release_stack_readiness.py
• python3 scripts/verify_release_stack_live_status.py --allow-missing-top-pr
• python3 scripts/verify_release_stack_lineage.py --allow-missing-top-pr --report-only
• python3 scripts/verify_release_stack_merge_runbook.py --allow-missing-top-pr --report-only
• python3 scripts/generate_platform_bundle_manifest.py --check
• python3 scripts/generate_platform_adoption_pack.py --check
• python3 scripts/verify_github_launch_operator_guide.py --check
• python3 scripts/verify_ecosystem_submission_pack.py
• python3 scripts/verify_platform_ecosystem_packs.py
• python3 scripts/verify_external_adoption.py --pack platform/generated/study-anything-platform-adoption-pack.zip --current-worktree
• python3 scripts/verify_published_image_evidence.py --check
• python3 scripts/verify_release_asset_adoption.py --fixture fixtures/release-asset-adoption/asset-only-pass.json --asset-dir platform/generated --runtime metadata-only
• ./scripts/release_check.sh
• git diff --check

Notes

• The runbook does not execute merges.
• It avoids deleting intermediate stacked branches and instructs maintainers to retarget the next PR to main after each merge.
• It does not persist GitHub tokens or live check payloads.