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/providers/registry/files.py

@@ -0,0 +1,31 @@
+# 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.
+
+from llama_stack.providers.datatypes import Api, InlineProviderSpec, ProviderSpec, RemoteProviderSpec
+from llama_stack.providers.utils.sqlstore.sqlstore import sql_store_pip_packages
+
+
+def available_providers() -> list[ProviderSpec]:
+    return [
+        InlineProviderSpec(
+            api=Api.files,
+            provider_type="inline::localfs",
+            # TODO: make this dynamic according to the sql store type
+            pip_packages=sql_store_pip_packages,
+            module="llama_stack.providers.inline.files.localfs",
+            config_class="llama_stack.providers.inline.files.localfs.config.LocalfsFilesImplConfig",
+            description="Local filesystem-based file storage provider for managing files and documents locally.",
+        ),
+        RemoteProviderSpec(
+            api=Api.files,
+            provider_type="remote::s3",
+            adapter_type="s3",
+            pip_packages=["boto3"] + sql_store_pip_packages,
+            module="llama_stack.providers.remote.files.s3",
+            config_class="llama_stack.providers.remote.files.s3.config.S3FilesImplConfig",
+            description="AWS S3-based file storage provider for scalable cloud file management with metadata persistence.",
+        ),
+    ]