feat: Create sqlrooms notebook package - #131

Introduce @sqlrooms/notebook package to enable an interactive, reactive notebook experience for SQLRooms, supporting SQL, markdown, and input cells with automatic dependency-based re-execution.

The reactive execution model is a core design principle, where changes in SQL cell definitions or input variable values automatically trigger re-runs of dependent SQL cells. This fosters an exploratory data analysis workflow by ensuring results are always up-to-date with the latest data and parameters. SQL cells create named views in a dedicated notebook schema for easy referencing and isolation.

Summary by CodeRabbit

• New Features

  • Notebook example app with Sheets panel, Assistant drawer, data-source upload, and a Sheets-based UI for notebook/canvas switching.
  • Extensible cell registry with default sql/text/vega/input cells; new cell UIs (SQL run/cancel with paginated results, Vega charts, text, inputs) and Add‑new controls.

• Bug Fixes

  • More reliable SQL execution with retry/backoff and debounced persistence to reduce lost state.

• Refactor

  • Unified cells architecture: cells/DAG shared between Notebook and Canvas for consistent behavior and rendering.

• Tests

  • New tests for DAG execution, sheet ownership, qualification, and concurrency.

• Docs

  • New READMEs and example app configs.