Add pull request creation skill

[clement-ux]

Summary

• add a repository-local pull request skill shared by Codex and Claude from a single source
• enforce person-prefixed branches, conventional commits, safe pushes, relevant labels, self-assignment, and history-based PR titles
• define the smart-contract deployment PR format using [ARM] Deploy 039 USDC ARM on mainnet #315 as the reference

Verification

• validated .agents/skills/create-pull-request
• validated .claude/skills/create-pull-request
• confirmed the Claude path resolves to the canonical Codex skill
• git diff --check