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>

tests/e2e/README.md

@@ -6,14 +6,14 @@ To be able to run e2e tests successfully run_e2e_script prepares environment in
 
 1. build and start weather_forecast demo (using docker compose)
 1. build, install and start model server async (using uv)
-1. build and start arch gateway (using docker compose)
+1. build and start Plano gateway (using docker compose)
 1. wait for model server to be ready
-1. wait for arch gateway to be ready
+1. wait for Plano gateway to be ready
 1. start e2e tests (using uv)
    1. runs llm gateway tests for llm routing
    2. runs prompt gateway tests to test function calling, parameter gathering and summarization
 2. cleanup
-   1. stops arch gateway
+   1. stops Plano gateway
    2. stops model server
    3. stops weather_forecast demo