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/ui-unit-tests.yml

@@ -8,7 +8,7 @@ on:
   pull_request:
     branches: [ main ]
     paths:
-      - 'llama_stack/ui/**'
+      - 'src/llama_stack/ui/**'
       - '.github/workflows/ui-unit-tests.yml' # This workflow
   workflow_dispatch:
 
@@ -33,22 +33,22 @@ jobs:
         with:
           node-version: ${{ matrix.node-version }}
           cache: 'npm'
-          cache-dependency-path: 'llama_stack/ui/package-lock.json'
+          cache-dependency-path: 'src/llama_stack/ui/package-lock.json'
 
       - name: Install dependencies
-        working-directory: llama_stack/ui
+        working-directory: src/llama_stack/ui
         run: npm ci
 
       - name: Run linting
-        working-directory: llama_stack/ui
+        working-directory: src/llama_stack/ui
         run: npm run lint
 
       - name: Run format check
-        working-directory: llama_stack/ui
+        working-directory: src/llama_stack/ui
         run: npm run format:check
 
       - name: Run unit tests
-        working-directory: llama_stack/ui
+        working-directory: src/llama_stack/ui
         env:
           CI: true