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.

docs/4-implementation/scheduler/jobq.md

@@ -34,7 +34,7 @@ swarm controller and hive-c0re each build their own graph on it.
 Two views, one per graph, drawn by the same component: the swarm UI's
 `/jobs` page shows the swarm controller's graph, and the hive dashboard's
 **R3BU1LD QU3U3** tab (see
-[`2-handbook/web-ui/dashboard.md`](../../2-handbook/web-ui/dashboard.md)) shows that hive's. Each
+[`2-handbook/hives/dashboard.md`](../../2-handbook/hives/dashboard.md)) shows that hive's. Each
 **row** is one job; the rows nested under it are that job's steps, in
 order (occasionally a couple run side by side). A step shows one of: