swarm-ui: let the create-agent form set the new agent's initial AGENTS.md

The form gets a collapsed "instructions" section with a plain text box. Its
text goes in the optional agents_md field of the create request, and is left
out while the box is empty or only whitespace. The controller's size refusal
shows under the box instead of in the form's general error panel. The text
box empties after a successful create.

The send and refusal logic sits in its own module with tests, and
docs/swarm/ui.md describes the field.

frontend/packages/swarm-ui/src/pages/CreateAgentForm.css

@@ -69,6 +69,11 @@
 .create-agent-result-ok {
   color: var(--green);
 }
+/* The controller's refusal of the AGENTS.md text, under its box. */
+.create-agent-field-error {
+  margin: 0;
+  color: var(--red);
+}
 /* The picked archetype's description, under its select. */
 .create-agent-archetype-description {
   margin: 0;