[codex] Add nosbook platform upload bridge

[ColinLi98]

Summary

• Add POST /v1/author/nosbooks/import to import .nosbook/v1 envelopes as Author private drafts.
• Add scripts/upload_nosbook.py for Codex-style agent uploads from either an existing .nosbook file or --local-work-id local Studio export.
• Document the API/CLI workflow and add targeted API, service, CLI, and docs coverage.

Validation

• .venv/bin/python -m py_compile scripts/upload_nosbook.py tests/test_nosbook_platform_upload.py
• .venv/bin/python -m pytest tests/test_nosbook_platform_upload.py tests/test_frontend_shell_docs.py -q (18 passed)

Notes

• Import result is always private_draft; this does not auto-publish or submit for review.
• The previous local end-to-end smoke runner was intentionally reverted and is not part of this PR.
• No generation kernel, planner, choice semantics, quality gate, or .nosbook/v1 schema changes are included.