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.

nix/host-modules/hive-c0re/options.nix

@@ -234,7 +234,7 @@ in
         `hivectl` (and thus the whole hive) without `sudo`.
 
         This is a real privilege grant: the admin socket is *full* hive
-        control — spawn / kill / destroy / deploy — see `docs/2-handbook/trust-boundary/boundary.md`.
+        control — spawn / kill / destroy / deploy — see `docs/2-handbook/hives/boundary.md`.
         Keep the list to trusted operators. Empty (the default) keeps the
         socket root-only, as before.
       '';