feat: explain inaccessible client upload paths

[adity982]

요약

Detect ChatGPT-style /mnt/user-data/... upload paths before normal workspace resolution and return a typed, redacted validation error with actionable Korean guidance to save the file locally and pass its real path.

관련 이슈

Fixes #75

변경 종류

• 버그 수정
• 기능
• 문서
• 리팩터링

체크리스트

• Focused workspace/protocol tests pass (4 parametrized cases)
• ruff check --select E9,F passes for changed modules/tests
• scripts/check_public_hygiene.py passes
• Full Windows pytest run: 5 existing platform-specific failures (path trailing-space semantics, symlink privilege, and POSIX mode assertions)
• CHANGELOG.md updated (not needed for this focused error-handling change)
• 문서 갱신이 필요한지 확인했습니다.