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 .`

llama_stack/ui/components/chat-playground/prompt-suggestions.tsx

@@ -1,28 +0,0 @@
-interface PromptSuggestionsProps {
-  label: string;
-  append: (message: { role: "user"; content: string }) => void;
-  suggestions: string[];
-}
-
-export function PromptSuggestions({
-  label,
-  append,
-  suggestions,
-}: PromptSuggestionsProps) {
-  return (
-    <div className="space-y-6">
-      <h2 className="text-center text-2xl font-bold">{label}</h2>
-      <div className="flex gap-6 text-sm">
-        {suggestions.map(suggestion => (
-          <button
-            key={suggestion}
-            onClick={() => append({ role: "user", content: suggestion })}
-            className="h-max flex-1 rounded-xl border bg-background p-4 hover:bg-muted"
-          >
-            <p>{suggestion}</p>
-          </button>
-        ))}
-      </div>
-    </div>
-  );
-}