The agenthub marketplace entry was added in commit 2f57ef8 but dropped
when PR #369 (code-to-prd) merged with a conflict in marketplace.json.
Re-adds the entry with all cross-domain keywords and 7 slash commands.
Total plugins: 22 (was 21)
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>