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/apis/inference/event_logger.py

@@ -1,43 +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 termcolor import cprint
-
-from llama_stack.apis.inference import (
-    ChatCompletionResponseEventType,
-    ChatCompletionResponseStreamChunk,
-)
-
-
-class LogEvent:
-    def __init__(
-        self,
-        content: str = "",
-        end: str = "\n",
-        color="white",
-    ):
-        self.content = content
-        self.color = color
-        self.end = "\n" if end is None else end
-
-    def print(self, flush=True):
-        cprint(f"{self.content}", color=self.color, end=self.end, flush=flush)
-
-
-class EventLogger:
-    async def log(self, event_generator):
-        async for chunk in event_generator:
-            if isinstance(chunk, ChatCompletionResponseStreamChunk):
-                event = chunk.event
-                if event.event_type == ChatCompletionResponseEventType.start:
-                    yield LogEvent("Assistant> ", color="cyan", end="")
-                elif event.event_type == ChatCompletionResponseEventType.progress:
-                    yield LogEvent(event.delta, color="yellow", end="")
-                elif event.event_type == ChatCompletionResponseEventType.complete:
-                    yield LogEvent("")
-            else:
-                yield LogEvent("Assistant> ", color="cyan", end="")
-                yield LogEvent(chunk.completion_message.content, color="yellow")