Backend: publish handover memory to NotebookLM

[hikari716]

Context

The Backend protocol (src/agent_handover/backends.py) lets handover state be
published anywhere. A NotebookLM backend would let teams query their handover
history in NotebookLM alongside other sources.

Task

Add a NotebookLMBackend implementing publish(message, dry_run=False) -> bool
that uploads/refreshes the memory/ snapshot as a NotebookLM source. Auth and
notebook id via config/env. Plain-Markdown store remains the source of truth.

Notes

• Larger, integration-flavored task — design discussion welcome before a PR.

Pointers

• src/agent_handover/backends.py (Protocol + GitBackend for reference)