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/samples_python/weather_forecast/run_demo.sh

@@ -72,8 +72,8 @@ start_demo() {
     exit 1
   fi
 
-  # Step 4: Start Arch
-  echo "Starting Arch with config.yaml..."
+  # Step 4: Start Plano
+  echo "Starting Plano with config.yaml..."
   planoai up config.yaml
 
   # Step 5: Start Network Agent with the chosen Docker Compose file
@@ -91,8 +91,8 @@ stop_demo() {
     docker compose -f "$compose_file" down
   done
 
-  # Stop Arch
-  echo "Stopping Arch..."
+  # Stop Plano
+  echo "Stopping Plano..."
   planoai down
 }