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 .`

.github/workflows/pre-commit.yml

@@ -41,11 +41,11 @@ jobs:
         with:
           node-version: '20'
           cache: 'npm'
-          cache-dependency-path: 'llama_stack/ui/'
+          cache-dependency-path: 'src/llama_stack/ui/'
 
       - name: Install npm dependencies
         run: npm ci
-        working-directory: llama_stack/ui
+        working-directory: src/llama_stack/ui
 
       - name: Run pre-commit
         id: precommit