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/batches/reference/config.py

@@ -0,0 +1,40 @@
+# 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 pydantic import BaseModel, Field
+
+from llama_stack.core.storage.datatypes import KVStoreReference
+
+
+class ReferenceBatchesImplConfig(BaseModel):
+    """Configuration for the Reference Batches implementation."""
+
+    kvstore: KVStoreReference = Field(
+        description="Configuration for the key-value store backend.",
+    )
+
+    max_concurrent_batches: int = Field(
+        default=1,
+        description="Maximum number of concurrent batches to process simultaneously.",
+        ge=1,
+    )
+
+    max_concurrent_requests_per_batch: int = Field(
+        default=10,
+        description="Maximum number of concurrent requests to process per batch.",
+        ge=1,
+    )
+
+    # TODO: add a max requests per second rate limiter
+
+    @classmethod
+    def sample_run_config(cls, __distro_dir__: str) -> dict:
+        return {
+            "kvstore": KVStoreReference(
+                backend="kv_default",
+                namespace="batches",
+            ).model_dump(exclude_none=True),
+        }