`sandbox.network.allowedDomains` not enforced for Bash tool or WebFetch

## Summary

`sandbox.network.allowedDomains` is accepted by the SDK but has no enforcement
effect on Bash subprocess network calls or WebFetch requests.

## Steps to Reproduce

Configure sandbox:
{ "sandbox": { "enabled": true, "network": { "allowedDomains": ["*.walmart.com"] } } }

Run: curl https://google.com (via Bash tool)
Or: WebFetch https://google.com

## Expected
Both blocked. Error returned for non-allowed domains.

## Actual
Both succeed. Restriction silently ignored.

## Root Cause
No OS-level enforcement exists — no proxy, no pf rules, no seccomp/bubblewrap,
no Darwin Sandbox. Bash subprocesses have full network access regardless of config.

The SDK docs say "use WebFetch permission rules for network" but WebFetch is also
not blocked. It's unclear what allowedDomains actually enforces.

## SDK version: 0.2.120 / macOS

## Request
Either implement real enforcement or document clearly that Bash network is unrestricted.