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/inline/files/localfs/config.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 typing import Any
+
+from pydantic import BaseModel, Field
+
+from llama_stack.core.storage.datatypes import SqlStoreReference
+
+
+class LocalfsFilesImplConfig(BaseModel):
+    storage_dir: str = Field(
+        description="Directory to store uploaded files",
+    )
+    metadata_store: SqlStoreReference = Field(
+        description="SQL store configuration for file metadata",
+    )
+    ttl_secs: int = 365 * 24 * 60 * 60  # 1 year
+
+    @classmethod
+    def sample_run_config(cls, __distro_dir__: str) -> dict[str, Any]:
+        return {
+            "storage_dir": "${env.FILES_STORAGE_DIR:=" + __distro_dir__ + "/files}",
+            "metadata_store": SqlStoreReference(
+                backend="sql_default",
+                table_name="files_metadata",
+            ).model_dump(exclude_none=True),
+        }