docs(setup): 📝 duplicate GitLab settings table into README and NEXT STEPS, add duplications rule
The Settings → General → Merge requests table in MIGRATION-GUIDE.md wasn't reachable from a generated project (that file isn't copied into the blueprint). Duplicate the full table into README.md's Repository Setup section, add a pointer from the post_gen_project.py NEXT STEPS message, and note the source-of-truth relationship in MIGRATION-GUIDE.md.
Add .agent/rules/duplications.md documenting this and the existing push/bypass-instructions duplication between README.md and post_gen_project.py, so future edits keep all copies in sync.