Publish curated release notes before generated PR history

[flyingrobots]

Problem

The generated v19.0.0 GitHub Release notes listed intermediate PR titles such as the superseded openWarp/Timeline prototypes before explaining the final supported Runtime/Lane contract. The release had to be edited after publication to prepend the authoritative contract and retained-v18 migration warning.

Generated history is useful evidence, but it is not a safe first-use release narrative for a train that intentionally replaced intermediate designs before the tag.

Acceptance criteria

• Extract or supply the dated version synopsis from CHANGELOG.md when creating a GitHub Release.
• Put breaking changes and required migration steps before generated PR history.
• Preserve the registry publication summary and generated comparison list as supplemental evidence.
• Make release-workflow reruns idempotent without deleting a curated synopsis.
• Add a workflow-level regression test or fixture covering a superseded intermediate PR title.

[coderabbitai]

🔗 Related PRs

#108 - Plan post-v18 release handoff and next goalpost [merged]
#674 - Prepare v18.1.0 release [merged]
#687 - docs: formalize Continuum release contract [merged]
#710 - Release 18.2.1 [merged]
#802 - Release v19.0.0 [open]

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