docs: the handbook follows the three layers: swarm, hives, agents

The handbook's sections were agent-lifecycle, integrations, swarm, tools,
trust-boundary and web-ui, which do not follow how the system is built, so a
reader could not tell which layer a page is about. Its pages now sit in
swarm/, hives/ and agents/, with the accounts an agent can hold outside the
system in external-accounts/.

- swarm/: forge, matrix and the knowledge repo join the swarm pages, and so
  does the swarmctl reference.
- hives/: the dashboard, hivectl and its reference, and the operator/agent
  boundary, with a README that takes over the old web-ui and tools pages.
- agents/: the roster, persistence, config changes, the agent's own web page
  (was web-ui/agent) and the security model, with a README.
- external-accounts/: GitHub and GitLab.

Pages are moved whole; none is split yet. Every reference follows: relative
links in the docs, `docs/2-handbook/...` paths in code comments, the nix
checks that regenerate the CLI references and the ignore list, and docs/README.md,
whose handbook section is now by layer.

swarm-controller/src/vcs_metrics.rs

@@ -1,6 +1,6 @@
 //! OTEL export of commit/push activity across every forge repo in the
 //! instance — the metric forgejo's own native `/metrics` endpoint (see
-//! `docs/2-handbook/integrations/forge.md`'s note on the survey that shipped it) does not carry.
+//! `docs/2-handbook/swarm/forge.md`'s note on the survey that shipped it) does not carry.
 //! That endpoint gives issue/comment/repo counts because Forgejo tracks
 //! those as durable rows it can `SELECT COUNT(*)` on demand; a commit or a
 //! push is not stored anywhere as a row to count, only observed in passing