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/env.py

@@ -0,0 +1,24 @@
+# Copyright (c) Meta Platforms, Inc. and affiliates.
+# All rights reserved.
+#
+# This source code is licensed under the terms described in the LICENSE file in
+# the root directory of this source tree.
+
+import os
+
+
+class MissingCredentialError(Exception):
+    pass
+
+
+def get_env_or_fail(key: str) -> str:
+    """Get environment variable or raise helpful error"""
+    value = os.getenv(key)
+    if not value:
+        raise MissingCredentialError(
+            f"\nMissing {key} in environment. Please set it using one of these methods:"
+            f"\n1. Export in shell: export {key}=your-key"
+            f"\n2. Create .env file in project root with: {key}=your-key"
+            f"\n3. Pass directly to pytest: pytest --env {key}=your-key"
+        )
+    return value