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/providers/remote/inference/cerebras/config.py

@@ -1,30 +0,0 @@
-# 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
-from typing import Any
-
-from pydantic import Field
-
-from llama_stack.providers.utils.inference.model_registry import RemoteInferenceProviderConfig
-from llama_stack.schema_utils import json_schema_type
-
-DEFAULT_BASE_URL = "https://api.cerebras.ai"
-
-
-@json_schema_type
-class CerebrasImplConfig(RemoteInferenceProviderConfig):
-    base_url: str = Field(
-        default=os.environ.get("CEREBRAS_BASE_URL", DEFAULT_BASE_URL),
-        description="Base URL for the Cerebras API",
-    )
-
-    @classmethod
-    def sample_run_config(cls, api_key: str = "${env.CEREBRAS_API_KEY:=}", **kwargs) -> dict[str, Any]:
-        return {
-            "base_url": DEFAULT_BASE_URL,
-            "api_key": api_key,
-        }