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 .`

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"

scripts/unit-tests.sh

@@ -27,4 +27,4 @@ fi
 
 # Run unit tests with coverage
 uv run --python "$PYTHON_VERSION" --with-editable . --group unit \
-    coverage run --source=llama_stack -m pytest -s -v tests/unit/ "$@"
+    coverage run --source=src/llama_stack -m pytest -s -v tests/unit/ "$@"