v0.9.12 Maintenance Release

This maintenance release fixes query conversation isolation and prevents empty answers from being reported as successful results.

Added

• Query callers can pass new_conversation=True to start an independent conversation without looking up the notebook's persistent chat.
• The CLI exposes the same behavior through --new-conversation.

Fixed

• Empty, whitespace-only, or missing query answers now raise a structured service error.
• The REPL /clear command now starts a genuinely fresh conversation.
• Existing default behavior remains unchanged: omitted conversation IDs continue the notebook's persistent chat when available.

Verification

• Full test suite: 1,415 passed, 39 skipped.
• Ruff lint and formatting checks passed.
• Live CLI query with --new-conversation returned a cited answer and a distinct conversation ID from the notebook's existing active session.