security: remove hardcoded credentials, untrack .env

- config/system_config.py: defaults are localhost with empty credentials;
  real hosts/secrets must come from env (.env or docker-compose)
- main.py: help text lists env var names instead of real NATS host/user
- doormile_test.py: reads infra config from env instead of literals
- untrack .env, ignore .env/.env.*, add .env.example with keys only
- pytest.ini: testpaths=tests so doormile_test.py isn't collected

Credentials remain in git history and must be rotated.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_012AJLYcbTHCe45fyFnMfEin

.env.example

@@ -0,0 +1,16 @@
+GO_API_BASE_URL=
+NATS_URL=
+REDIS_HOST=
+REDIS_PORT=
+REDIS_PASSWORD=
+INTERNAL_API_KEY=
+DB_HOST=
+DB_PORT=
+DB_NAME=
+DB_USER=
+DB_PASSWORD=
+NATS_USER=
+NATS_PASSWORD=
+ANTHROPIC_API_KEY=
+LLM_MODEL=claude-opus-4-8
+LOG_LEVEL=INFO