refactor: simplify paired review system and add reviewer container isolation

Phase 1 — Strip over-engineering (-3,665 lines):
- DB tables: 7 → 3 (remove executions, approvals, artifacts, events)
- Task statuses: 11 → 5 (active, review_ready, in_review, merge_ready, completed)
- Remove plan governance, event sourcing, gate/verdict, freshness guards
- paired-execution-context: 980 → 299 lines
- Session commands: remove /risk, /plan, /approve-plan, /request-plan-changes

Phase 2 — Container isolation for reviewers:
- Add container-runtime.ts (Docker abstraction)
- Add credential-proxy.ts (API key injection without container exposure)
- Add container-runner.ts (reviewer-specific read-only mount + tmpfs)
- Add container/Dockerfile + agent-runner (Chromium, Claude Code, Codex)
- agent-runner.ts: auto-route reviewer execution to container mode

container/Dockerfile

@@ -0,0 +1,63 @@
+# EJClaw Reviewer Container
+# Runs Claude/Codex agent in read-only isolated environment for code review.
+
+FROM node:22-slim
+
+# System dependencies for Chromium (E2E tests, screenshots)
+RUN apt-get update && apt-get install -y \
+    chromium \
+    fonts-liberation \
+    fonts-noto-cjk \
+    fonts-noto-color-emoji \
+    libgbm1 \
+    libnss3 \
+    libatk-bridge2.0-0 \
+    libgtk-3-0 \
+    libx11-xcb1 \
+    libxcomposite1 \
+    libxdamage1 \
+    libxrandr2 \
+    libasound2 \
+    libpangocairo-1.0-0 \
+    libcups2 \
+    libdrm2 \
+    libxshmfence1 \
+    curl \
+    git \
+    && rm -rf /var/lib/apt/lists/*
+
+# Chromium paths for Playwright/agent-browser
+ENV AGENT_BROWSER_EXECUTABLE_PATH=/usr/bin/chromium
+ENV PLAYWRIGHT_CHROMIUM_EXECUTABLE_PATH=/usr/bin/chromium
+
+# Install Claude Code and Codex CLI globally
+RUN npm install -g @anthropic-ai/claude-code @openai/codex
+
+# Create app directory
+WORKDIR /app
+
+# Copy and install agent runner
+COPY agent-runner/package*.json ./
+RUN npm install
+COPY agent-runner/ ./
+RUN npm run build
+
+# Create workspace directories
+RUN mkdir -p /workspace/project /workspace/group /workspace/global \
+    /workspace/ipc/messages /workspace/ipc/input
+
+# Create entrypoint script
+RUN printf '#!/bin/bash\nset -e\ncat > /tmp/input.json\nnode /app/dist/index.js < /tmp/input.json\n' \
+    > /app/entrypoint.sh && chmod +x /app/entrypoint.sh
+
+# Set ownership to node user (non-root)
+RUN chown -R node:node /workspace /home/node
+
+# Switch to non-root user
+USER node
+
+# Working directory is the read-only project mount
+WORKDIR /workspace/project
+
+# Entry point reads JSON from stdin, outputs JSON to stdout
+ENTRYPOINT ["/app/entrypoint.sh"]