feat(cli): add supabase notebooks pull

[SaxonF]

Adds the supabase notebooks command family and its first subcommand, notebooks pull. Notebooks live in supabase/notebooks/<name>.json, one committed JSON file each, alongside supabase/functions/ and supabase/compute/.

supabase notebooks pull                     # write notebooks missing locally
supabase notebooks pull <notebook-id>       # replace one local copy

Identity is the name, not the id

The API assigns each notebook a uuid, but a checkout is shared through git and a uuid in a filename is unreadable — so the file name is the notebook's identity across the two sides. Two consequences the code is shaped around:

• Notebook names are free text up to 255 characters, so a name can hold a path separator, a Windows device name, or a trailing dot. Those are reported and skipped, never sanitised: a sanitised name would push back later as a rename of somebody's notebook. Portable-filename aliases (Sales vs sales, NFC vs NFD) are refused before anything is written, so a checkout stays valid when it moves between filesystems.
• Names are not unique in the API, so a project holding two notebooks of one name is refused rather than guessed at.

Reconciliation

Without an argument the checkout wins: only notebooks missing locally are written, and existing files are left alone. A notebook id is the opt-in overwrite path, and reconciles nothing — the argument says which notebook to replace, so the rest of the directory is not that invocation's business.

Local files naming no project notebook are the divergence the command asks about. Three answers rather than a confirmation, because the two useful ones point opposite ways: such a file is either one somebody deleted in the dashboard, or one somebody added locally and never pushed, and nothing in the list says which. An unattended run — non-TTY, machine output format, or a cancelled prompt — reports the divergence and leaves both sides alone rather than resolving it in a direction nobody chose.

Reviewer notes

• Files are published through a temp file plus a hard link, so an interrupted pull leaves no partial file and never clobbers a file that appeared after the inventory was read. pull/SIDE_EFFECTS.md records the full side-effect surface.
• The list walk follows the server's links.next cursor rather than counting rows, and fails closed on a missing or repeated cursor instead of looping.
• notebooks.shared.ts is deliberately the shared half for the whole family; notebooks push lands on top of this PR in the stack and reuses it.