feat: recursive upload to sync a folder as a page tree

[blaipr]

Problem

download --recursive exports a page tree, but there is no way back: pushing a folder of Markdown files means one upload/edit invocation per file, by hand, in the right order.

Change

Adds upload --recursive <folder> <parent_url>: every <name>.md becomes a page under the parent and sibling <name>/ folders recurse as children — exactly the layout recursive download writes. Per file, the title comes from the leading # h1 (filename fallback, underscores to spaces), and the target page is picked by version marker first, then title lookup, else a new page is created. Marker version conflicts skip the file with a warning unless --force. Local images are attached per page as in single uploads. The title positional becomes optional (validated as required in non-recursive mode).

Compatibility

Single-file upload behavior is unchanged. upload gains --recursive/--force flags.

Checklist

• ruff check src tests, ruff format --check src tests, python -m pytest tests/ pass locally
• Tests added (fake-client hierarchy, update-by-title, marker guard, --force, filename fallback)
• Docs updated (docs/usage.md upload section)
• CHANGELOG.md updated under Unreleased