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.

frontend/packages/shared/src/side-panel/hive-side-panel.js

@@ -31,7 +31,7 @@ import { el } from "../dom.js";
 import { attachShadowCss } from "../shadow-css.js";
 import sidePanelCss from "./hive-side-panel.css";
 
-// See docs/2-handbook/web-ui/agent.md::Side panel for the hit-strip + pointer-capture +
+// See docs/2-handbook/agents/web-page.md::Side panel for the hit-strip + pointer-capture +
 // localStorage persistence model; CSS clamps the stored value to min
 // 320px / max 96vw and out-of-range stored values are dropped silently.
 // Shared by both packages deliberately — see module header.