fix: show actual memory usage (exclude cache) + add build:container script

- Dashboard memory: use /proc/meminfo MemAvailable instead of os.freemem()
  to exclude Linux buffer/cache from reported usage
- Add build:container npm script for reviewer Docker image rebuild
- Update deploy commands in CLAUDE.md and README.md to include build:container

package.json

@@ -7,6 +7,7 @@
   "scripts": {
     "build": "tsc",
     "build:runners": "bun install --cwd runners/agent-runner && bun run --cwd runners/agent-runner build && bun install --cwd runners/codex-runner && bun run --cwd runners/codex-runner build",
+    "build:container": "docker build -f container/Dockerfile -t ejclaw-reviewer:latest .",
     "restart:stack": "bash scripts/restart-ejclaw-stack.sh",
     "restart:hint": "bun src/restart-context-cli.ts write",
     "start": "bun dist/index.js",