feat: chat persistence — save and resume Ask sessions

[rajnavakoti]

Description

Save Ask page conversations as persistent sessions that can be resumed later. Each session stores the question history, answers, and retrieval traces. Sessions are stored per-block.

Acceptance Criteria

• Each Ask conversation gets a session ID
• Questions and answers saved to session file (JSON)
• Session list visible in Ask page sidebar
• Click a session to resume with full history
• cb sessions list shows sessions in CLI
• Sessions include retrieval traces for later analysis
• Sessions scoped to the active block

Out of Scope

• Session sharing between users
• Session export
• Saving sessions as KB explorations (P2)