swarm-ui: make agent creation a five-step dialog with a fixed header and footer

The create dialog was one tall form that scrolled as a whole, so its header
and button moved out of view. It is now steps you go through in order:
identity, instructions (AGENTS.md in a box of its own), permissions,
resources and inference, then a summary of what will be sent and what the
agent creation queues, where create is. The header, step tabs and footer stay
put and only the step's own fields scroll. A refused AGENTS.md text sends the
dialog back to its step. The request is unchanged.

frontend/packages/swarm-ui/src/ui/form-field/FormField.css

@@ -4,7 +4,7 @@
    whichever container the caller gives it — the kit itself has no
    opinion on a maximum width; a page that wants one narrower than its
    own layout caps it at the layout level (`CreateAgentForm.css`'s
-   `.create-agent-form-col` is the existing example), same reasoning
+   `.create-agent-panel` is the existing example), same reasoning
    `Panel` has no width opinion of its own either. `min-height` is a
    touch-target floor (44px at the default 16px root font — WCAG
    2.5.5's minimum), not a visual choice — it's the same on every