docs(mnp): the protocol reference, brought to the cleanup push

MESHBAY_NODE_PROTOCOL.md and playlists.md move into the repository, where a
reader can follow them.

The protocol reference was written against the tree before the cleanup landed
and described three things that no longer exist. Corrected here rather than
left to be discovered:

  * member_upload is gone from the handshake ack and from the message
    catalogue. Whether a member may write is a property of each root, and a
    summary field beside the authoritative one is a second source for one
    question — whichever the reader consults first decides it.
  * video_root, audio_root and photo_roots are gone from the ack, the signed-op
    table and the message reference. app_directories is not "the general form"
    of three narrower ops any more; it is the only one. What survives is their
    storage key on the node, because that is a key on an operator's disk rather
    than on the wire, and a node upgraded into this has to find its own
    configuration.
  * The sender-key implementation the chat section pointed at has been deleted,
    along with the ratchet. The argument for deriving a key per device stands on
    its own now instead of pointing at a module to compare against.

playlists.md is added as written — a design for a feature that is not built, and
the first one to need per-account state spanning several groups on several
nodes. Its decided shape is summarised in MESHBAY_DESIGN.md section 9.10.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01YVoHVCcfBqud6ZjG4db3y7

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