feat: sandbox database integration

[SimoneBottoni]

This PR adds hosted sandbox database management to the CLI and refactors the multi-agent cookbook to use automated payload construction via as_tool() extractors.

Key Changes

Features

• Sandbox management: new CLI commands (sandbox create, sandbox status, sandbox delete) for hosted sandbox databases with automatic health checks and recreation on expiry during bootstrap.
• Automated payload construction: multi-agent cookbook uses agent.as_tool() with custom_output_extractor to build HandoffPayloads automatically, removing manual wrapper functions.

Refactoring

• Multi-agent cookbook: specialist agents converted from factory functions + wrapper tools to as_tool() pattern, reducing boilerplate.
• Doctor command: detects sandbox status and reports database type (sandbox vs personal).