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/registry/agents.py

@@ -1,41 +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.
-
-
-from llama_stack.providers.datatypes import (
-    Api,
-    InlineProviderSpec,
-    ProviderSpec,
-)
-from llama_stack.providers.utils.kvstore import kvstore_dependencies
-
-
-def available_providers() -> list[ProviderSpec]:
-    return [
-        InlineProviderSpec(
-            api=Api.agents,
-            provider_type="inline::meta-reference",
-            pip_packages=[
-                "matplotlib",
-                "pillow",
-                "pandas",
-                "scikit-learn",
-                "mcp>=1.8.1",
-            ]
-            + kvstore_dependencies(),  # TODO make this dynamic based on the kvstore config
-            module="llama_stack.providers.inline.agents.meta_reference",
-            config_class="llama_stack.providers.inline.agents.meta_reference.MetaReferenceAgentsImplConfig",
-            api_dependencies=[
-                Api.inference,
-                Api.safety,
-                Api.vector_io,
-                Api.tool_runtime,
-                Api.tool_groups,
-                Api.conversations,
-            ],
-            description="Meta's reference implementation of an agent system that can use tools, access vector databases, and perform complex reasoning tasks.",
-        ),
-    ]