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/pages/LinkMatrixAccountForm.css

@@ -1,10 +1,7 @@
 /* <LinkMatrixAccountForm> — rendered inside a `Dialog` from `AgentsPage`,
-   one panel wide (no second info column — this form has no "what this
-   creates" explanation worth a whole sibling card the way
-   `CreateAgentForm` does, just the one-line note already in the JSX).
-   Column layout + `.link-matrix-account-result-ok` mirror
-   `CreateAgentForm.css`'s `.create-agent-form`/`.create-agent-result-ok`
-   exactly — same shape, kept as its own file rather than sharing that
+   one panel wide. Column layout + `.link-matrix-account-result-ok`
+   mirror `CreateAgentForm.css`'s `.create-agent-fields`/`.create-agent-result-ok`
+   — same shape, kept as its own file rather than sharing that
    one since "form" here means a different component's layout, not a
    generic kit primitive (see that file's own top comment on why
    `ui/form-field` stays the only truly shared piece). */