Reference the goalpost when opening release PRs

[flyingrobots]

Problem

The v19.0.0 release PR was opened without its completed release-goalpost issue reference. The required PR Issue Reference check failed until the PR body was manually updated with Related: #758; no code change was needed.

Acceptance criteria

• The release runbook or release-PR helper requires the exact same-repository goalpost issue number.
• The generated release PR body includes that reference before CI starts.
• A local or unit-level check exercises the same accepted reference forms as the hosted policy.
• Ordinary non-release PR behavior is unchanged.

[coderabbitai]

🔗 Related PRs

#108 - Plan post-v18 release handoff and next goalpost [merged]
#650 - Reconcile v18 issue taxonomy [merged]
#676 - release: repair v18.1.1 autotag link gate [merged]
#687 - docs: formalize Continuum release contract [merged]
#701 - Fix: clarify release branch check workflow name [merged]

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