feat: replace AtlasChat with assistant-ui Thread component

- Install @assistant-ui/react and @assistant-ui/react-markdown
- components/assistant-ui/thread.tsx — full Thread UI with primitives
- components/assistant-ui/markdown-text.tsx — GFM markdown renderer
- AtlasChat.tsx — useLocalRuntime adapter calling existing atlas-chat API

Gives proper markdown rendering, branch switching, copy/retry actions,
cancel button during streaming, and a polished thread experience.

Made-with: Cursor

components/assistant-ui/markdown-text.tsx

@@ -0,0 +1,50 @@
+"use client";
+
+import { MarkdownTextPrimitive } from "@assistant-ui/react-markdown";
+import remarkGfm from "remark-gfm";
+import type { FC } from "react";
+import { cn } from "@/lib/utils";
+
+export const MarkdownText: FC = () => {
+  return (
+    <MarkdownTextPrimitive
+      remarkPlugins={[remarkGfm]}
+      className="prose prose-sm dark:prose-invert max-w-none leading-relaxed"
+      components={{
+        h1: ({ className, ...props }) => (
+          <h1 className={cn("text-base font-bold mt-3 mb-1", className)} {...props} />
+        ),
+        h2: ({ className, ...props }) => (
+          <h2 className={cn("text-sm font-bold mt-3 mb-1", className)} {...props} />
+        ),
+        h3: ({ className, ...props }) => (
+          <h3 className={cn("text-sm font-semibold mt-2 mb-1", className)} {...props} />
+        ),
+        p: ({ className, ...props }) => (
+          <p className={cn("mb-2 last:mb-0", className)} {...props} />
+        ),
+        ul: ({ className, ...props }) => (
+          <ul className={cn("list-disc list-outside ml-4 mb-2 space-y-0.5", className)} {...props} />
+        ),
+        ol: ({ className, ...props }) => (
+          <ol className={cn("list-decimal list-outside ml-4 mb-2 space-y-0.5", className)} {...props} />
+        ),
+        li: ({ className, ...props }) => (
+          <li className={cn("leading-relaxed", className)} {...props} />
+        ),
+        strong: ({ className, ...props }) => (
+          <strong className={cn("font-semibold", className)} {...props} />
+        ),
+        code: ({ className, ...props }) => (
+          <code className={cn("bg-muted px-1 py-0.5 rounded text-xs font-mono", className)} {...props} />
+        ),
+        pre: ({ className, ...props }) => (
+          <pre className={cn("bg-muted rounded-lg p-3 overflow-x-auto text-xs my-2", className)} {...props} />
+        ),
+        blockquote: ({ className, ...props }) => (
+          <blockquote className={cn("border-l-2 border-muted-foreground/30 pl-3 italic text-muted-foreground my-2", className)} {...props} />
+        ),
+      }}
+    />
+  );
+};