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/page.tsx

@@ -0,0 +1,7 @@
+"use client";
+
+import { ResponsesTable } from "@/components/responses/responses-table";
+
+export default function ResponsesPage() {
+  return <ResponsesTable paginationOptions={{ limit: 20 }} />;
+}