fix(sandbox): preserve NEKO_* env across sudo re-exec in start-browser.sh

[xfgong]

What

When start-browser.sh runs as the agent (uid 1000), it re-execs through
sudo so the chown/lock/pidfile steps can touch /var/run. The sandbox
image bakes NEKO_WEBRTC_ICESERVERS_* (TURN at turn:192.168.1.208:3478)
into Config.Env so Neko can relay WebRTC media, but /etc/sudoers sets
Defaults env_reset and does not include NEKO_* in env_keep, so sudo
strips the ICE config. Neko falls back to Google STUN only, the WebRTC
media path has no relay, and the live view white-screens.

Why

The agent path (browser skill telling the agent to run
/usr/local/bin/start-browser.sh as the sandbox user) loses the TURN
server baked into the image. The backend's start_browser flow uses
as_root=True and was never affected; the bug only shows up when the
agent starts the browser itself. Surfaced once PR #506
(feat/2026-08-19-auto-open-browser-preview) started opening the Browser
panel automatically and the agent-driven path became user-visible.

How

Preserve the NEKO_* vars across the exec sudo boundary via a temp file
the root branch sources back. NEKO_ENV_FILE is set on the sudo command
line (not the inherited env) so it survives env_reset and points the
re-exec'd root shell at the file.

start_browser (backend, as_root=True) takes the existing fast path and
never hits the preservation code, so behaviour for the live-view flow is
unchanged.

Test plan

After rebuilding the cubeplex-sandbox image with a new tag and rolling
it onto a node:

1. Restart a sandbox; verify docker inspect on the new tag still bakes
   NEKO_WEBRTC_ICESERVERS_BACKEND/FRONTEND with turn:192.168.1.208:3478.
2. As the agent (sudo -u cubeplex), run /usr/local/bin/start-browser.sh;
   confirm /var/log/neko/neko.log shows
   iceservers-backend=[{"urls":["turn:192.168.1.208:3478"],...}] (not
   the Google STUN default).
3. Open the Browser panel; confirm the Neko stream renders non-white.
4. Repeat with a sandbox on the other TURN-less node (217) to confirm the
   ICE relay still works through coturn on 208.

Requires rebuilding cubeplex-sandbox (new tag) and deploying it for the
fix to take effect; sh -n and a code review suffice at the repo level.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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