chore: strengthen release readiness

[rogerchappel]

Summary

• complete npm package metadata and release/support pack contents
• add a release notes starter and release checklist where missing
• document release verification commands in the README
• add a support policy for issue and security-report routing

Verification

PASS npm run package:smoke --if-present
Test and build	pass	22s	https://github.com/rogerchappel/gitbriefcase/actions/runs/27175050741/job/80222082910	
dry-run	pass	17s	https://github.com/rogerchappel/gitbriefcase/actions/runs/27175050830/job/80222082940