refactor(mnp)!: one answer to "may this member write", and it is the root

The group-wide `member_upload` switch is gone: the message, the signed
operation, the field on the handshake ack, the `upload` alias on every root in
the index payload, and the client's fallback path to it.

Whether a member may write has been a property of each root for a while, and
that is the model that survives: a single flag over the group cannot express
"this library is published read-only and that folder is a drop box", which is
the ordinary arrangement. What was left of the switch was a handler that logged
a deprecation and acted on nothing, and a client that read `ack.member_upload`
whenever the roots carried no `writable` — a second source for one question,
with whichever the code consulted first deciding it.

`roots.describe()` drops `upload` for the same reason: it was `writable` under
an older name, and two names for one boolean is one too many.

The paperclip now says "nowhere to write" rather than picking a root, in a group
that has none writable. That is the honest answer; the fallback picked whatever
came first and failed at send time.

Node suite 1215 passed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01AsoWC3GmhNdwVFomW3QjH3

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