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/cli/llama.py

@@ -0,0 +1,56 @@
+# 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.
+
+import argparse
+
+from llama_stack.log import setup_logging
+
+from .stack import StackParser
+from .stack.utils import print_subcommand_description
+
+
+class LlamaCLIParser:
+    """Defines CLI parser for Llama CLI"""
+
+    def __init__(self):
+        self.parser = argparse.ArgumentParser(
+            prog="llama",
+            description="Welcome to the Llama CLI",
+            add_help=True,
+            formatter_class=argparse.RawTextHelpFormatter,
+        )
+
+        # Default command is to print help
+        self.parser.set_defaults(func=lambda args: self.parser.print_help())
+
+        subparsers = self.parser.add_subparsers(title="subcommands")
+
+        # Add sub-commands
+        StackParser.create(subparsers)
+
+        print_subcommand_description(self.parser, subparsers)
+
+    def parse_args(self) -> argparse.Namespace:
+        args = self.parser.parse_args()
+        if not isinstance(args, argparse.Namespace):
+            raise TypeError(f"Expected argparse.Namespace, got {type(args)}")
+        return args
+
+    def run(self, args: argparse.Namespace) -> None:
+        args.func(args)
+
+
+def main():
+    # Initialize logging from environment variables before any other operations
+    setup_logging()
+
+    parser = LlamaCLIParser()
+    args = parser.parse_args()
+    parser.run(args)
+
+
+if __name__ == "__main__":
+    main()