feat(notebooks): add --markdown to get, create, update, and edit - #750

Summary

Work with a notebook as a Markdown document instead of a JSON cells array:

pup notebooks get 12345 --markdown                      # print as Markdown
pup notebooks create --markdown --file doc.md           # create; prints the new id
pup notebooks update 12345 --markdown --file doc.md     # replace the document
pup notebooks edit 12345 --markdown --file fragment.md  # append server-side

These hit /api/unstable/notebooks, not yet promoted to /api/v2, so the flags are marked experimental in --help.

Changes

• create --markdown prints the new notebook id, not the document. The id only exists in the JSON:API representation — per ADR-006 it is resource identity and deliberately absent from Markdown frontmatter — so create negotiates JSON:API and leaves reading the document to get:

  ID=$(pup notebooks create --markdown --file doc.md)
  pup notebooks get "$ID" --markdown

• edit --markdown appends via the content-fragment endpoint. The JSON path fetches, splices, and rewrites the whole notebook; this does not.

• --jq is rejected with --markdown, keyed on the flag rather than cfg.jq so an inherited PUP_FILTER does not block extension subprocesses.

• JSON files passed with --markdown are rejected before any request.

• Errors distinguish reads from writes. A notebook with no Markdown projection gives an actionable message; on a write, and on any failure to decode a 2xx response, the error says the change may already have landed, since retrying would duplicate a create or re-append a fragment.

• Raw-text print helper moved to util_ext::print_text_document; skills remote get had the same inline idiom.

Testing

• All four commands, request bodies and media types, content-type and empty-body guards, JSON-file and BOM-prefixed-JSON rejection, and the error-translation paths for reads, writes, and post-success decode failures.
• Against the live API: create returns an id that get resolves, edit appends without disturbing existing content, update replaces, and a cells-era notebook gives the translated error rather than a raw 5xx.

Known gaps

• Rich-text notebooks only. Notebooks from the older cells API have no Markdown projection.
• update --markdown is lossy. Replaces the whole document, drops anything Markdown cannot represent. No prompt, matching JSON update.
• No conflict detection. document_revision is returned but not enforced on any write path.
• No targeted edits. update replaces, edit appends; no way to change one section in place.
• --output and agent mode do nothing under --markdown, matching skills remote get.