docs: user guide and conventions catch up with per-node identity

USERGUIDE said registration submits your public keys "so other members can wrap
GEK bundles for you". Both halves are wrong now: registration creates an account
and nothing else, and nobody wraps anything for a key fetched from the hub. The
API reference and the register body followed the same correction.

CLAUDE.md gains the block a future session needs before touching registration or
anything shaped like a user's public key: keys are born at first contact with a
node and stay there, the hub publishes none, tokens carry no pk_user, and a
scripted signup is now a real account.

Left alone deliberately: first-review.md, docs/poc-v1*.md and poc/spike-results.md
still describe the old JWT and registration. They are records of what was true on
their date, like second-review's verdict table, and draft-v5 is what states the
present.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

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