chore(package): migrate to src/ layout

Moved package code from llama_stack/ to src/llama_stack/ following Python
packaging best practices. Updated pyproject.toml, MANIFEST.in, and tool
configurations accordingly.

Public API and import paths remain unchanged. Developers will need to
reinstall in editable mode after pulling this change.

Also updated paths in pre-commit config, scripts, and GitHub workflows.

scripts/check-init-py.sh

@@ -16,7 +16,7 @@ if (( BASH_VERSINFO[0] < 4 )); then
     exit 1
 fi
 
-PACKAGE_DIR="${1:-llama_stack}"
+PACKAGE_DIR="${1:-src/llama_stack}"
 
 if [ ! -d "$PACKAGE_DIR" ]; then
     echo "ERROR: Package directory '$PACKAGE_DIR' does not exist"

scripts/distro_codegen.py

@@ -93,7 +93,7 @@ def pre_import_distros(distro_dirs: list[Path]) -> None:
 
 
 def main():
-    distros_dir = REPO_ROOT / "llama_stack" / "distributions"
+    distros_dir = REPO_ROOT / "src" / "llama_stack" / "distributions"
     change_tracker = ChangedPathTracker()
 
     with Progress(

scripts/run-ui-linter.sh

@@ -6,7 +6,7 @@
 # the root directory of this source tree.
 
 set -e
-cd llama_stack/ui
+cd src/llama_stack/ui
 
 if [ ! -d node_modules ] || [ ! -x node_modules/.bin/prettier ] || [ ! -x node_modules/.bin/eslint ]; then
   echo "UI dependencies not installed, skipping prettier/linter check"