Rename all arch references to plano across the codebase

Complete rebrand from "Arch"/"archgw" to "Plano" including:
- Config files: arch_config_schema.yaml, workflow, demo configs
- Environment variables: ARCH_CONFIG_* → PLANO_CONFIG_*
- Python CLI: variables, functions, file paths, docker mounts
- Rust crates: config paths, log messages, metadata keys
- Docker/build: Dockerfile, supervisord, .dockerignore, .gitignore
- Docker Compose: volume mounts and env vars across all demos/tests
- GitHub workflows: job/step names
- Shell scripts: log messages
- Demos: Python code, READMEs, VS Code configs, Grafana dashboard
- Docs: RST includes, code comments, config references
- Package metadata: package.json, pyproject.toml, uv.lock

External URLs (docs.archgw.com, github.com/katanemo/archgw) left as-is.

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

demos/use_cases/preference_based_routing/docker-compose.yaml

@@ -8,14 +8,14 @@ services:
       - "12000:12000"
       - "12001:12001"
     environment:
-      - ARCH_CONFIG_PATH=/app/arch_config.yaml
+      - PLANO_CONFIG_PATH=/app/plano_config.yaml
       - OPENAI_API_KEY=${OPENAI_API_KEY:?OPENAI_API_KEY environment variable is required but not set}
       - ANTHROPIC_API_KEY=${ANTHROPIC_API_KEY:?ANTHROPIC_API_KEY environment variable is required but not set}
       - OTEL_TRACING_GRPC_ENDPOINT=http://host.docker.internal:4317
       - OTEL_TRACING_ENABLED=true
       - RUST_LOG=debug
     volumes:
-      - ./config.yaml:/app/arch_config.yaml:ro
+      - ./config.yaml:/app/plano_config.yaml:ro
       - /etc/ssl/cert.pem:/etc/ssl/cert.pem
 
   anythingllm: