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/agent/src/agent.css

@@ -16,7 +16,7 @@
 
 /* ─── full-screen layout overrides ─────────────────────────────────
    The agent page mounts a full-viewport terminal under a fixed
-   frosted-glass header. See docs/2-handbook/web-ui/agent.md::Per-agent
+   frosted-glass header. See docs/2-handbook/agents/web-page.md::Per-agent
    page for the layer / pill / side-panel structure; rules below
    override the in-page defaults from @hive/shared. */
 
@@ -184,7 +184,7 @@ h3 {
 }
 .agent-icon {
   /* Square identity anchor — explicit 5em sizing + align-self.
-     See docs/2-handbook/web-ui/agent.md::Per-agent page (Agent icon) for the
+     See docs/2-handbook/agents/web-page.md::Per-agent page (Agent icon) for the
      intrinsic-dim-pushes-parent-flex-open + sticks-to-top
      rationale. */
   width: 5em;
@@ -594,7 +594,7 @@ pre.diff {
 /* Tail pill (↓ N new): z-index bumped above the floating chrome so the
    pill sits on the top layer instead of being clipped by it. (Pill is anchored in
    `.agent-main` rather than `.terminal-wrap` — see
-   docs/2-handbook/web-ui/agent.md::Per-agent page Terminal-wrap paragraph for the
+   docs/2-handbook/agents/web-page.md::Per-agent page Terminal-wrap paragraph for the
    backdrop-filter stacking-context rationale.) */
 .agent-main .tail-pill {
   bottom: 0.6em;