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.

swarm-controller/README.md

@@ -123,7 +123,7 @@ The swarm's control plane. The swarm UI and `swarmctl` are its clients.
   icons, the cross-repo issue report, and the UI's quick links.
 - **Alerts** — the swarm Grafana's current alerts (`GET /api/alerts`), read
   at request time with a Viewer service-account token from the secret store
-  → [`docs/2-handbook/swarm/credentials-reference.md`](../docs/2-handbook/swarm/credentials-reference.md).
+  → [`docs/4-implementation/swarm/credentials-reference.md`](../docs/4-implementation/swarm/credentials-reference.md).
 
 It reads its configuration once at startup, from the environment the nix module
 sets. It keeps the webhook HMAC secret in the secret store and

swarm-controller/src/matrix_account.rs

@@ -2,7 +2,7 @@
 //!
 //! **The internal one** — [`agent_token`] — is the agent's own `main` account on
 //! the swarm's homeserver, minted with the swarm's appservice token and stored
-//! where the agent's daemon reads it. See docs/2-handbook/swarm/credentials-reference.md for why
+//! where the agent's daemon reads it. See docs/4-implementation/swarm/credentials-reference.md for why
 //! `main` is the one name [`put_matrix_account`] refuses to write.
 //! [`hive_sender`] mints each **hive's** sender account the same way, sharing this module's helpers with [`agent_token`].
 //! [`operator_admin`] makes authelia's operators homeserver admins with the same