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/app/logs/responses/layout.tsx

@@ -0,0 +1,16 @@
+"use client";
+
+import React from "react";
+import LogsLayout from "@/components/layout/logs-layout";
+
+export default function ResponsesLayout({
+  children,
+}: {
+  children: React.ReactNode;
+}) {
+  return (
+    <LogsLayout sectionLabel="Responses" basePath="/logs/responses">
+      {children}
+    </LogsLayout>
+  );
+}