Write BEP for merging keypair resource policy into user resource policy

Write the BEP for merging keypair resource policy into user resource policy. Author it in the backend.ai worktree using the bep-guide skill: reserve a BEP number in proposals/README.md (BEP Number Registry) and draft from proposals/BEP-0000-template.md.

MANDATORY AUTHORING GATE: write the draft in Korean FIRST, get the user review/approval on the Korean draft, and ONLY THEN translate to English and open the PR. Never draft in English first or open the PR before the Korean review.

## Success Criteria
- [ ] BEP number reserved in proposals/README.md (BEP Number Registry)
- [ ] Korean draft written first and reviewed/approved by the user before any English
- [ ] proposals/BEP-XXXX-keypair-user-resource-policy.md drafted from the template (English, post-review)
- [ ] PR submitted for BEP discussion

JIRA Issue: BA-7032

Start by reading the bep-guide skill, the BEP Number Registry in proposals/README.md, and proposals/BEP-0000-template.md. Reserve a number and write the Korean draft first for user review and approval; only then translate it into proposals/BEP-XXXX-keypair-user-resource-policy.md and submit the PR for discussion.

Written by the indexing model from the issue text.