chore(validator): drop .env, keep all config in FastAgent YAMLs

OPENAI_BASE_URL was duplicated between .env and fastagent.config.yaml;
the YAML is authoritative, so .env is dead weight. Removing the .env
template and gitignore entry, updating README to reflect.

The real fastagent.secrets.yaml stays gitignored;
fastagent.secrets.yaml.example remains as the documented schema.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

validator/.gitignore

@@ -1,5 +1,4 @@
 fastagent.secrets.yaml
-.env
 .venv/
 *.egg-info/
 __pycache__/