chore: improve and rename release skill to release-a2a

[kabir]

Summary

• Rename release → release-a2a to avoid ambiguity with generic plugins
• Detect upstream vs fork remotes dynamically from git remote -v instead of hardcoding origin
• Push PR branches to fork with --repo/--head flags for gh pr create
• Fix Javadoc output path (docs/public/dev/apidocs, not target/reports/apidocs)
• Combine docs and SNAPSHOT bump into a single post-release PR to avoid redundant CI waits
• Verify local main is in sync with upstream before starting