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/2-handbook/swarm/schedules.md

@@ -41,7 +41,7 @@ kqzpmwab  "standup"  every 1d  from 2026-10-05T07:00:00Z  next 2026-10-05T07:00:
 - `schedule list` shows one row per prompt: id, name, cadence, start, next
   fire time, agents, and the first line of the body.
 
-The full flag reference is [`swarmctl-cli.md`](../tools/swarmctl-cli.md).
+The full flag reference is [`swarmctl-cli.md`](swarmctl-cli.md).
 
 ## When a prompt fires