Chromium fails to launch on hosts with unprivileged userns disabled (No usable sandbox) — breaks browse/design/make-pdf #2157

Summary

On Linux hosts where unprivileged user namespaces are disabled (default on Ubuntu 23.10+ / 24.04 via AppArmor, and common in containers/CI), the bundled Chromium refuses to launch because gstack starts it without --no-sandbox. Every skill that drives Chromium fails: /browse (screenshot/goto), /design-consultation + /plan-design-review (mockups), and /make-pdf.

gstack version: 1.58.5.0

Error

From make-pdf/dist/pdf setup (same launch path as browse):

[pid=...][err] [FATAL:content/browser/zygote_host/zygote_host_impl_linux.cc:128]
No usable sandbox! If you are running on Ubuntu 23.10+ or another Linux distro
that has disabled unprivileged user namespaces with AppArmor, see
.../apparmor-userns-restrictions.md ... If you want to live dangerously and need
an immediate workaround, you can try using --no-sandbox.
    at launch (browse/src/browser-manager.ts:372:35)
    at async start (browse/src/server.ts:2974:28)

Environment

• Linux (Ubuntu 23.10+ / kernel with kernel.apparmor_restrict_unprivileged_userns=1)
• Playwright Chromium present at ~/.cache/ms-playwright/chromium-*/chrome-linux/chrome and chromium_headless_shell-*
• chromium also present but snap-confined (writes to a private /tmp, cannot read dotdirs like ~/.gstack), so it is not a usable fallback either.

Repro

1. On an Ubuntu 24.04 host (or any host with sysctl kernel.apparmor_restrict_unprivileged_userns=1).
2. Run any Chromium-backed skill, e.g. ~/.claude/skills/gstack/make-pdf/dist/pdf setup, or browse/dist/browse goto file:///tmp/x.html && browse screenshot /tmp/x.png.
3. Launch fails with the FATAL sandbox error above; no screenshot/PDF/mockup is produced.

Root cause

browser-manager.ts launches Chromium without --no-sandbox, and the host cannot create the sandbox via unprivileged user namespaces.

Suggested fixes (any one unblocks it)

1. Detect-and-fallback (preferred): on Linux, probe whether the sandbox can start; if not, relaunch with --no-sandbox and emit a one-line warning. This is what Puppeteer/Playwright wrappers commonly do for CI/containers.
2. Opt-in env: honor something like GSTACK_CHROMIUM_NO_SANDBOX=1 (and/or a general GSTACK_CHROMIUM_ARGS) passed through to the launch args. Today neither the browse nor make-pdf binary exposes a no-sandbox flag or arg passthrough.
3. Docs: if --no-sandbox is undesirable by default, document the host-side fix in setup output: sudo sysctl kernel.apparmor_restrict_unprivileged_userns=0 (non-persistent) or an AppArmor profile.

Impact

Hits three skill families on affected hosts (browse, design mockups, make-pdf). Workaround today is to bypass the gstack binaries entirely and drive the Playwright chrome directly with --no-sandbox (e.g. chrome --headless=new --no-sandbox --print-to-pdf), which loses the gstack styling/daemon.