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/core/common.sh

@@ -0,0 +1,37 @@
+#!/usr/bin/env bash
+
+# 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.
+
+cleanup() {
+  # For venv environments, no special cleanup is needed
+  # This function exists to avoid "function not found" errors
+  local env_name="$1"
+  echo "Cleanup called for environment: $env_name"
+}
+
+handle_int() {
+  if [ -n "$ENVNAME" ]; then
+    cleanup "$ENVNAME"
+  fi
+  exit 1
+}
+
+handle_exit() {
+  if [ $? -ne 0 ]; then
+    echo -e "\033[1;31mABORTING.\033[0m"
+    if [ -n "$ENVNAME" ]; then
+      cleanup "$ENVNAME"
+    fi
+  fi
+}
+
+
+
+# check if a command is present
+is_command_available() {
+  command -v "$1" &>/dev/null
+}