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/ui/.prettierrc

@@ -0,0 +1,10 @@
+{
+  "semi": true,
+  "trailingComma": "es5",
+  "singleQuote": false,
+  "printWidth": 80,
+  "tabWidth": 2,
+  "useTabs": false,
+  "bracketSpacing": true,
+  "arrowParens": "avoid"
+}