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>

coder/workspace.Dockerfile

@@ -0,0 +1,23 @@
+FROM codercom/enterprise-base:latest
+
+USER root
+
+# Node.js 22
+RUN curl -fsSL https://deb.nodesource.com/setup_22.x | bash - \
+    && apt-get install -y nodejs \
+    && rm -rf /var/lib/apt/lists/*
+
+# Global npm tools
+RUN npm install -g wrangler @anthropic-ai/claude-code
+
+# Playwright system dependencies (both Ubuntu 24.04 and 22.04 package names)
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    libnss3 libnspr4 libatk1.0-0 libatk-bridge2.0-0 \
+    libdrm2 libxkbcommon0 libxcomposite1 libxdamage1 libxfixes3 \
+    libxrandr2 libgbm1 libpango-1.0-0 libcairo2 libatspi2.0-0 \
+    jq netcat-openbsd \
+    && (apt-get install -y libcups2t64 libasound2t64 2>/dev/null \
+        || apt-get install -y libcups2 libasound2 2>/dev/null) \
+    && rm -rf /var/lib/apt/lists/*
+
+USER coder