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

[andyjmorgan]

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 from Markdown
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

• 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.
• The 5xx for notebooks with no Markdown projection becomes an actionable message. On writes it says the outcome is unknown — the render runs after the mutation, so a failure does not prove the write was rejected.
• 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, both error-translation paths.
• Against the live API: get returns frontmatter plus body, create creates, 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.