credentials.md drops the target-shape framing and keeps the rules, where a
secret lives, the swarm- and hive-level secrets and how to add one. The queue
responder's keys, per-principal identities, how a reader reaches the store, the
store UI's login and where the root lives move to
4-implementation/swarm/credentials.md. The per-secret reference moves to
4-implementation/swarm/credentials-reference.md. Links and comments follow.
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