feat(chatbook): add Convert to Chatbook for ordinary notebooks

[pjdoland]

Summary

This adds the reverse of Export as code notebook: a way to turn an existing notebook into a Chatbook, as requested in #504. Ordinary notebooks get a Convert to Chatbook toolbar button and a Convert notebook to Chatbook palette command. Both write a Chatbook copy beside the original and open it. Every code cell arrives as a Cd cell, ready to switch to natural language.

Solution

What the copy contains

• The copy is named <stem>-chatbook.ipynb, numbered when that name is taken, following the export's -python naming. It uses the export's collision-safe save, which is now shared as saveNotebookCopy.
• It is built from what is in the editor, so unsaved changes are included. The original is not changed.
• Code cells get nbi.chatbook = {mode: 'code', origin: 'code', codeSource, generatedCode} and run exactly as written.
• Everything else is kept: outputs, execution counts, cell ids and tags, attachments, markdown and raw cells, and other notebook metadata. Only the kernelspec and language_info change to Chatbook's. When the source names its kernel, the name is recorded under nbi.chatbook.sourceKernel.

English descriptions

• Conversion sends nothing to a model.
• The existing path generates a cell's one-line English description on its first successful run, and the dialog discloses this.
• A description the cell already carries is kept only when its metadata ties it to that exact code. That is the case for a Chatbook that was exported and is converted back, so its prompts survive the round trip.
• Natural-language cells of a Chatbook whose kernel was switched away are left as natural language, provided they have run and have not been edited since.

Language check

• The Chatbook execution kernel is one setting shared by every Chatbook, so conversion compares the notebook's language with it first.
• Same language: the notebook converts, with a note when its kernel differs.
• Unknown language: the notebook converts, with a note naming the kernel it will run on.
• Different language (an R notebook while the backend is Python): the dialog explains why the notebook can't be converted yet and, when a suitable kernel is installed, offers Open Chatbook settings. It never changes that setting itself, because the change would also affect the user's existing Chatbooks.

Visibility

• The button shows only on loaded, non-Chatbook notebooks, and only while Chatbook is enabled.
• It is re-checked when the kernelspec metadata changes, so a notebook switched to or from the Chatbook kernel updates correctly.
• The icon is the Chatbook kernel's own glyph (VscChatSparkle, MIT).

Testing

• tests/ts/chatbook.test.ts: the conversion contract. It covers:
  • what is kept;
  • when a description survives, including after a failed refresh, an edited prompt, and code edited after generation;
  • a Chatbook → export → Chatbook round trip;
  • mode switching on converted cells;
  • naming;
  • language normalization;
  • the fit matrix.
• tests/ts/chatbook-convert.test.ts: drives convertNotebookToChatbook with a fake Contents service. It checks that a taken name is numbered rather than overwritten, the source is not modified, and no summarize call or fetch happens.
• tests/ts/chatbook-toolbar-convert.test.ts: covers button visibility, every dialog branch (convert, cancel, unknown language, blocked with and without an installed kernel), one conversion at a time, and a failed save.
• The new guards were mutation-checked: removing any one of them fails a test.
• pytest tests/ -q (2233 passed), jlpm tsc --noEmit, jlpm lint:check, jlpm jest (53 suites, 614 tests).
• Checked in a running JupyterLab 4.6:
  • Converted a Python notebook with outputs and tags. The copy opened on the Chatbook kernel with Cd badges and its outputs, and the saved metadata matched the above.
  • Converting again produced -chatbook-1 with the "Created Chatbook copy" toast.
  • Running a converted cell generated its description.
  • An R notebook was blocked, and Open Chatbook settings opened the Chatbook tab.
  • The button stayed hidden on the Chatbook tabs.
  • No console errors.

Risks / follow-ups

• Existing export behavior noticed while testing, unchanged here, filed as Export as code notebook drops the code of a Cd cell switched to natural language #509. A Cd cell switched to NL and not re-run exports as a comment of its English, so its code does not reach the exported notebook.
• Natural-language cells of a kernel-switched Chatbook that were never run, or were edited since their last run, carry nothing that distinguishes them from code, so they convert as code. The docs recommend switching such a Chatbook back to the Chatbook kernel instead.
• Possible later additions, if they seem worthwhile:
  • a file-browser context-menu entry;
  • a per-notebook backend kernel (the recorded sourceKernel would feed it);
  • generating English on convert, off by default;
  • an admin policy for code summaries.
• The button is inserted after the existing Chatbook buttons, so like them it relies on the cellType toolbar item being present.

Screenshots

Closes #504