release: create v0.5.1 tag and GitHub Release

[DecisionNerd]

Objective

When the candidate is eligible, create the immutable v0.5.1 tag and GitHub Release.

Done when

• ci: keep v0.5.1 publication automation small and resumable #288 has produced the candidate record.
• The tag points to that candidate and the GitHub Release has accurate notes and links.
• Record the release URL and candidate identity in release: ship GraphForge v0.5.1 with fast-fail recovery #192's final release record.

Non-goals

• Re-running candidate checks or publishing registry packages.
• Altering v0.5.0 history.