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/distributions/postgres-demo/__init__.py

@@ -1,7 +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 .postgres_demo import get_distribution_template  # noqa: F401

llama_stack/distributions/postgres-demo/postgres_demo.py

@@ -1,125 +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.apis.models import ModelType
-from llama_stack.core.datatypes import (
-    BuildProvider,
-    ModelInput,
-    Provider,
-    ShieldInput,
-    ToolGroupInput,
-)
-from llama_stack.distributions.template import (
-    DistributionTemplate,
-    RunConfigSettings,
-)
-from llama_stack.providers.inline.inference.sentence_transformers import SentenceTransformersInferenceConfig
-from llama_stack.providers.remote.inference.vllm import VLLMInferenceAdapterConfig
-from llama_stack.providers.remote.vector_io.chroma.config import ChromaVectorIOConfig
-from llama_stack.providers.utils.kvstore.config import PostgresKVStoreConfig
-from llama_stack.providers.utils.sqlstore.sqlstore import PostgresSqlStoreConfig
-
-
-def get_distribution_template() -> DistributionTemplate:
-    inference_providers = [
-        Provider(
-            provider_id="vllm-inference",
-            provider_type="remote::vllm",
-            config=VLLMInferenceAdapterConfig.sample_run_config(
-                url="${env.VLLM_URL:=http://localhost:8000/v1}",
-            ),
-        ),
-    ]
-    providers = {
-        "inference": [
-            BuildProvider(provider_type="remote::vllm"),
-            BuildProvider(provider_type="inline::sentence-transformers"),
-        ],
-        "vector_io": [BuildProvider(provider_type="remote::chromadb")],
-        "safety": [BuildProvider(provider_type="inline::llama-guard")],
-        "agents": [BuildProvider(provider_type="inline::meta-reference")],
-        "tool_runtime": [
-            BuildProvider(provider_type="remote::brave-search"),
-            BuildProvider(provider_type="remote::tavily-search"),
-            BuildProvider(provider_type="inline::rag-runtime"),
-            BuildProvider(provider_type="remote::model-context-protocol"),
-        ],
-    }
-    name = "postgres-demo"
-
-    vector_io_providers = [
-        Provider(
-            provider_id="${env.ENABLE_CHROMADB:+chromadb}",
-            provider_type="remote::chromadb",
-            config=ChromaVectorIOConfig.sample_run_config(
-                f"~/.llama/distributions/{name}",
-                url="${env.CHROMADB_URL:=}",
-            ),
-        ),
-    ]
-    default_tool_groups = [
-        ToolGroupInput(
-            toolgroup_id="builtin::websearch",
-            provider_id="tavily-search",
-        ),
-        ToolGroupInput(
-            toolgroup_id="builtin::rag",
-            provider_id="rag-runtime",
-        ),
-    ]
-
-    default_models = [
-        ModelInput(
-            model_id="${env.INFERENCE_MODEL}",
-            provider_id="vllm-inference",
-        )
-    ]
-    embedding_provider = Provider(
-        provider_id="sentence-transformers",
-        provider_type="inline::sentence-transformers",
-        config=SentenceTransformersInferenceConfig.sample_run_config(),
-    )
-    embedding_model = ModelInput(
-        model_id="nomic-embed-text-v1.5",
-        provider_id=embedding_provider.provider_id,
-        model_type=ModelType.embedding,
-        metadata={
-            "embedding_dimension": 768,
-        },
-    )
-    return DistributionTemplate(
-        name=name,
-        distro_type="self_hosted",
-        description="Quick start template for running Llama Stack with several popular providers",
-        container_image=None,
-        template_path=None,
-        providers=providers,
-        available_models_by_provider={},
-        run_configs={
-            "run.yaml": RunConfigSettings(
-                provider_overrides={
-                    "inference": inference_providers + [embedding_provider],
-                    "vector_io": vector_io_providers,
-                },
-                default_models=default_models + [embedding_model],
-                default_tool_groups=default_tool_groups,
-                default_shields=[ShieldInput(shield_id="meta-llama/Llama-Guard-3-8B")],
-                storage_backends={
-                    "kv_default": PostgresKVStoreConfig.sample_run_config(
-                        table_name="llamastack_kvstore",
-                    ),
-                    "sql_default": PostgresSqlStoreConfig.sample_run_config(),
-                },
-            ),
-        },
-        run_config_env_vars={
-            "LLAMA_STACK_PORT": (
-                "8321",
-                "Port for the Llama Stack distribution server",
-            ),
-        },
-    )