docs: the credentials page keeps the approach and moves the mechanics out

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.

docs/README.md

@@ -49,12 +49,13 @@ The services that exist once per swarm. Start at
   `services.hyperhive.agent.matrixAccounts`.
 - **What's `/knowledge`? How does the swarm-wide knowledge repo sync,
   and how to contribute a document?** → [`2-handbook/swarm/knowledge.md`](2-handbook/swarm/knowledge.md).
-- **Who mints each credential, who reads it, and how does it rotate — and
-  where's that shape headed?** → [`2-handbook/swarm/credentials.md`](2-handbook/swarm/credentials.md)
-  (the rules, where each secret goes and why) and
-  [`2-handbook/swarm/credentials-reference.md`](2-handbook/swarm/credentials-reference.md) (the
-  tables: each credential, current state and target state, and where each
-  file lives today).
+- **Who mints each credential, who reads it, and how does it rotate?** →
+  [`2-handbook/swarm/credentials.md`](2-handbook/swarm/credentials.md) (the
+  rules, where each secret goes and why, adding a secret) and
+  [`4-implementation/swarm/credentials-reference.md`](4-implementation/swarm/credentials-reference.md) (the
+  tables: each credential and where each file lives). The store's identities,
+  keys and login:
+  [`4-implementation/swarm/credentials.md`](4-implementation/swarm/credentials.md).
 - **How does an agent get a prompt at set times, and why doesn't it fire
   twice after downtime?** → [`2-handbook/swarm/schedules.md`](2-handbook/swarm/schedules.md).
 - **How to create an agent from a ready-made set of permissions and