swarm ui: the create form picks an archetype from tiles with their icons

An archetype's meta gets an optional icon, the SVG text (at most 16 KiB), which
the controller checks, stores and serves on GET /api/archetypes. The built-in
archetypes read theirs from nix/archetypes/icons. The create form shows each
archetype as a tile with its icon and name, laid out like the broadcast screen's
agent tiles, in place of the dropdown, and draws the SVG through an img data URL.

docs/2-handbook/swarm/archetypes.md

@@ -54,7 +54,10 @@ hyperhiveArchetypes.reviewer.meta.prompt = builtins.readFile ./prompts/reviewer.
   evaluates `meta` alone.
 - Every archetype needs a `description`. `permissions`, `limits` and
   `inference` are optional and take the shapes `POST /api/agents` takes.
-  `prompt` is optional text that prefills `AGENTS.md`, [below](#the-prompt). The swarm ignores
+  `prompt` is optional text that prefills `AGENTS.md`, [below](#the-prompt).
+  `icon` is optional SVG text, at most 16 KiB, that the create form shows on the
+  archetype's tile; `builtins.readFile ./icon.svg` gives it. The swarm refuses an
+  `icon` that isn't SVG text or is over the limit, like any bad `meta`, and ignores
   other keys in `meta`.
 - `module` is a NixOS module for the agent's container. The meta flake
   imports it into each agent created from the archetype.