Fix remaining arch references found in second pass

- config/docker-compose.dev.yaml: ARCH_CONFIG_FILE → PLANO_CONFIG_FILE,
  arch_config.yaml → plano_config.yaml, archgw_logs → plano_logs
- config/test_passthrough.yaml: container mount path
- tests/e2e/docker-compose.yaml: source file path (was still arch_config.yaml)
- cli/planoai/core.py: comment and log message
- crates/brightstaff/src/tracing/constants.rs: doc comment
- tests/{e2e,archgw}/common.py: get_arch_messages → get_plano_messages,
  arch_state/arch_messages variables renamed
- tests/{e2e,archgw}/test_prompt_gateway.py: updated imports and usages
- demos/shared/test_runner/{common,test_demos}.py: same renames
- tests/e2e/test_model_alias_routing.py: docstring
- .dockerignore: archgw_modelserver → plano_modelserver
- demos/use_cases/claude_code_router/pretty_model_resolution.sh: container name

Note: x-arch-* HTTP header values and Rust constant names intentionally
preserved for backwards compatibility with existing deployments.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/e2e/docker-compose.yaml

@@ -8,7 +8,7 @@ services:
       - "12000:12000"
       - "19901:9901"
     volumes:
-      - ../../demos/samples_python/weather_forecast/arch_config.yaml:/app/plano_config.yaml
+      - ../../demos/samples_python/weather_forecast/plano_config.yaml:/app/plano_config.yaml
       - /etc/ssl/cert.pem:/etc/ssl/cert.pem
     extra_hosts:
       - "host.docker.internal:host-gateway"