Custom workspace image + fix slash command invocation

- Add coder/workspace.Dockerfile with Node.js 22, wrangler, claude-code,
  and Playwright deps pre-installed. Eliminates ~3 min of installs on
  every workspace startup.
- Add CI workflow to build and push to registry.samson.media/coder-workspace
- Fix slash command: -p mode doesn't support /commands, so read the .md
  file directly, strip frontmatter, substitute $ARGUMENTS, pass as prompt
- Switch workspace image from codercom/enterprise-base to custom image

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.gitea/workflows/publish-workspace.yml

@@ -0,0 +1,27 @@
+name: Publish Workspace Image
+
+on:
+  push:
+    paths:
+      - 'coder/workspace.Dockerfile'
+    branches:
+      - main
+
+concurrency:
+  group: publish-workspace
+  cancel-in-progress: false
+
+jobs:
+  publish:
+    runs-on: self-hosted
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Log in to registry
+        run: echo "${{ secrets.REGISTRY_PASSWORD }}" | docker login registry.samson.media -u "${{ secrets.REGISTRY_USERNAME }}" --password-stdin
+
+      - name: Build and push
+        run: |
+          IMAGE=registry.samson.media/coder-workspace
+          docker build -f coder/workspace.Dockerfile -t "$IMAGE:latest" .
+          docker push "$IMAGE:latest"