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/interrupt-prompt.tsx

@@ -1,41 +0,0 @@
-"use client";
-
-import { AnimatePresence, motion } from "framer-motion";
-import { X } from "lucide-react";
-
-interface InterruptPromptProps {
-  isOpen: boolean;
-  close: () => void;
-}
-
-export function InterruptPrompt({ isOpen, close }: InterruptPromptProps) {
-  return (
-    <AnimatePresence>
-      {isOpen && (
-        <motion.div
-          initial={{ top: 0, filter: "blur(5px)" }}
-          animate={{
-            top: -40,
-            filter: "blur(0px)",
-            transition: {
-              type: "spring",
-              filter: { type: "tween" },
-            },
-          }}
-          exit={{ top: 0, filter: "blur(5px)" }}
-          className="absolute left-1/2 flex -translate-x-1/2 overflow-hidden whitespace-nowrap rounded-full border bg-background py-1 text-center text-sm text-muted-foreground"
-        >
-          <span className="ml-2.5">Press Enter again to interrupt</span>
-          <button
-            className="ml-1 mr-2.5 flex items-center"
-            type="button"
-            onClick={close}
-            aria-label="Close"
-          >
-            <X className="h-3 w-3" />
-          </button>
-        </motion.div>
-      )}
-    </AnimatePresence>
-  );
-}