Update docs/private submodule: Phase 21.4 plan
Phase 21.4詳細実装計画追加 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>
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Phase 21.4詳細実装計画追加 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <noreply@anthropic.com>