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>

docs/source/concepts/llm_providers/client_libraries.rst

@@ -54,7 +54,7 @@ The OpenAI SDK works with any provider through Plano's OpenAI-compatible endpoin
         base_url="http://127.0.0.1:12000/v1"
     )
 
-    # Use any model configured in your arch_config.yaml
+    # Use any model configured in your plano_config.yaml
     completion = client.chat.completions.create(
         model="gpt-4o-mini",  # Or use :ref:`model aliases <model_aliases>` like "fast-model"
         max_tokens=50,
@@ -231,7 +231,7 @@ The Anthropic SDK works with any provider through Plano's Anthropic-compatible e
         base_url="http://127.0.0.1:12000"
     )
 
-    # Use any model configured in your arch_config.yaml
+    # Use any model configured in your plano_config.yaml
     message = client.messages.create(
         model="claude-3-5-sonnet-20241022",
         max_tokens=50,