[Feat] Multiple chat sessions per notebook #97

Context

A notebook holds exactly one conversation, and SessionAutoSwitchService rolls it over silently when it approaches the context budget. The consequences:

• every topic in the notebook shares one thread, and its context,
• the user cannot start a genuinely separate line of inquiry,
• the automatic rollover is a backend event the user can neither see nor navigate.

PRODUCT.md already records this as confirmed by the maintainer. Open Notebook ships multiple chat sessions as a normal capability; it is the difference between "a chat box next to your documents" and a research workspace.

Proposal

Give a notebook many named sessions:

Notebook: Transformer
├── Understanding attention
├── Complexity derivation
├── Comparison with RNNs
└── Blog outline

• A session list beside the chat zone; + New chat creates one.
• Sessions are named manually, or derived from the first message.
• Each session keeps its own messages and its own retrieval scope, where fine-grained context is in play.
• Auto-rollover becomes visible: when a session rolls over, the user sees the archive boundary and can open the archived session. No silent context loss.
• Search across sessions from the session list (message search, not just titles).

Acceptance

• Multiple sessions per notebook, switchable without losing state
• Opening a notebook returns to the last-used session
• The auto-switch rollover is visible in the UI and the previous session is reachable
• Deleting a session is confirmed and does not delete the notebook or its sources
• Existing notebooks with a single session migrate without data loss

Verification

Create 3 sessions in one notebook, ask a question in each, restart the app, confirm all three survive with the right message history. Force a context rollover and confirm it is visible.

Depends on

The session/message schema change is additive. Provenance (#69) stores citations on messages, so citations must survive a session switch.