Add update subcommand to notebook_manager CLI

[chewinggum604-art]

Summary

NotebookLibrary.update_notebook() already exists on the class but has no
CLI front-end. To change a notebook's metadata today (description, topics,
tags, URL) you have to remove and re-add — which loses created_at,
use_count, and last_used.

This PR exposes update_notebook as a normal subcommand, mirroring the
shape of add:

python scripts/run.py notebook_manager.py update \
  --id my-notebook \
  --description "New description" \
  --topics "topic1,topic2,topic3"

All flags except --id are optional. Omitted flags leave the existing
value untouched. --topics and --tags replace the lists wholesale (same
semantics as add), which matches update_notebook()'s existing
behavior.

Diff

 scripts/notebook_manager.py | 22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

Pure addition — no existing behavior changes.