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>

.github/workflows/validate_plano_config.yml

@@ -0,0 +1,31 @@
+name: plano config tests
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+
+jobs:
+  validate_plano_config:
+    runs-on: ubuntu-latest
+    defaults:
+      run:
+        working-directory: .
+
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v3
+
+      - name: Set up Python
+        uses: actions/setup-python@v4
+        with:
+          python-version: "3.12"
+
+      - name: build plano docker image
+        run: |
+          docker build  -f Dockerfile . -t katanemo/plano -t katanemo/plano:0.4.6
+
+      - name: validate plano config
+        run: |
+          bash config/validate_plano_config.sh