Adds a generic ui/dialog/Dialog primitive (native <dialog>, no
third-party modal lib and no shadow-DOM custom element per the esbuild
gap on those) and wires a "+ agent" button into AgentsPage that opens
the existing create-agent form inside it, content unchanged from its
former life as a standalone /create-agent route/nav item.
Removes the "new agent" top-level nav entry and the /create-agent
route entirely -- creation now only reachable from the roster that
gets populated by it. CreateAgentPage.tsx/css renamed to
CreateAgentForm.tsx/css to match its new role as a mounted component
rather than a page.
Screenshot-verified the dialog open/closed states against a mock
server.
