chore(package): migrate to src/ layout (#3920)

Migrates package structure to src/ layout following Python packaging
best practices.

All code moved from `llama_stack/` to `src/llama_stack/`. Public API
unchanged - imports remain `import llama_stack.*`.

Updated build configs, pre-commit hooks, scripts, and GitHub workflows
accordingly. All hooks pass, package builds cleanly.

**Developer note**: Reinstall after pulling: `pip install -e .`

src/llama_stack/ui/components/layout/page-breadcrumb.tsx

@@ -0,0 +1,49 @@
+"use client";
+
+import Link from "next/link";
+import React from "react";
+import {
+  Breadcrumb,
+  BreadcrumbItem,
+  BreadcrumbLink,
+  BreadcrumbList,
+  BreadcrumbPage,
+  BreadcrumbSeparator,
+} from "@/components/ui/breadcrumb";
+
+export interface BreadcrumbSegment {
+  label: string;
+  href?: string;
+}
+
+interface PageBreadcrumbProps {
+  segments: BreadcrumbSegment[];
+  className?: string;
+}
+
+export function PageBreadcrumb({ segments, className }: PageBreadcrumbProps) {
+  if (!segments || segments.length === 0) {
+    return null;
+  }
+
+  return (
+    <Breadcrumb className={className}>
+      <BreadcrumbList>
+        {segments.map((segment, index) => (
+          <React.Fragment key={segment.label + index}>
+            <BreadcrumbItem>
+              {segment.href ? (
+                <BreadcrumbLink asChild>
+                  <Link href={segment.href}>{segment.label}</Link>
+                </BreadcrumbLink>
+              ) : (
+                <BreadcrumbPage>{segment.label}</BreadcrumbPage>
+              )}
+            </BreadcrumbItem>
+            {index < segments.length - 1 && <BreadcrumbSeparator />}
+          </React.Fragment>
+        ))}
+      </BreadcrumbList>
+    </Breadcrumb>
+  );
+}