feat: add storage doctor command

[cafitac]

Summary

• add read-only storage-doctor CLI plus audit-storage-layout alias for global-first learning storage observability
• report canonical AGENT_LEARNER_HOME, artifact counts, legacy/local source state, migration marker details, warnings, and suggested next commands
• expose the new commands through the npm wrapper help/completion and document them in README/changelog

Verification

• PYTHONPATH=src .venv/bin/python -m pytest tests/test_cli_bootstrap.py::test_storage_doctor_reports_global_counts_and_legacy_sources tests/test_cli_bootstrap.py::test_storage_doctor_is_read_only_for_unmigrated_local_source -q
• npm test
• PYTHONPATH=src .venv/bin/python -m pytest -q
• git diff --check