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/multi_turn_rag_agent/README.md

@@ -1,6 +1,6 @@
 # Multi-Turn Agentic Demo (RAG)
 
-This demo showcases how the **Arch** can be used to build accurate multi-turn RAG agent by just writing simple APIs.
+This demo showcases how **Plano** can be used to build accurate multi-turn RAG agent by just writing simple APIs.
 
 ![Example of Multi-turn Interaction](mutli-turn-example.png)
 
@@ -14,7 +14,7 @@ Provides information about various energy sources and considerations.
 
 # Starting the demo
 1. Please make sure the [pre-requisites](https://github.com/katanemo/arch/?tab=readme-ov-file#prerequisites) are installed correctly
-2. Start Arch
+2. Start Plano
    ```sh
    sh run_demo.sh
    ```