fix: render .env.example template in agentex init

The .env.example.j2 template file exists in every template directory but
was missing from the root_templates dict in init.py, so it was silently
dropped during project generation. Add it so generated projects include
a .env.example with LITELLM_API_KEY and OPENAI_BASE_URL placeholders,
guiding users to put credentials in .env instead of manifest.yaml.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: danielmillerp

src/agentex/lib/cli/commands/init.py

@@ -75,6 +75,7 @@ def create_project_structure(
     # Create root files
     root_templates = {
         ".dockerignore.j2": ".dockerignore",
+        ".env.example.j2": ".env.example",
         "manifest.yaml.j2": "manifest.yaml",
         "README.md.j2": "README.md",
         "environments.yaml.j2": "environments.yaml",