Define authenticated debug-console session contract

[Hrabovszki1023]

Closes #47

Summary

• define exact pylon> / pylon_debug> prompt parsing and fail-closed session modes
• require mode detection before login because repeated login is not idempotent
• require re-authentication after reconnect and best-effort verified logout on shutdown
• define secret handling, redaction, health/REST/Web exposure, deployment isolation and tests
• keep polling commands read-only and defer implementation to Implement authenticated debug-console session lifecycle #48

Verification

• python -m pytest — 207 passed
• python -m ruff check . — passed
• python -m mypy src tests — passed
• git diff --check — passed