feat(linux): route proxy-unaware HTTP traffic through managed proxy

[viyatb-oai]

Summary

• route proxy-unaware hostname-based IPv4 HTTP and HTTPS traffic through the existing managed proxy on Linux
• add a bounded synthetic DNS resolver and fail closed for uncaptured destinations
• verify activation is running outside the host network namespace
• grant namespace setup capabilities only to trusted setup code, then clear and verify every capability set before workload execution
• preserve the existing proxy-aware path when transparent setup is unsupported

This lets tools that do not honor proxy environment variables use managed egress without application-specific changes.

Compatibility

• proxy-aware clients continue to use the existing environment variables
• initial capture covers hostname-based IPv4 TCP 80/443 and UDP DNS
• raw IP destinations, IPv6, and DNS over TCP remain follow-ups
• transparent mode reserves local UDP 53 and TCP 80/443
• non-root setuid bubblewrap installations use the existing proxy-aware path
• loader-affecting environment variables are removed before privileged setup and are not restored to the workload
• missing resolver setup and unsupported proxy endpoints fall back to the existing proxy-aware path

Testing

• cargo check -p codex-linux-sandbox --target x86_64-unknown-linux-gnu --tests with the local Zig target wrappers
• cargo clippy -p codex-linux-sandbox --target x86_64-unknown-linux-gnu --tests --no-deps -- -D warnings with the local Zig target wrappers
• just test -p codex-linux-sandbox builds on macOS but has no runnable tests because this crate's tests are Linux-only
• git diff --check
• direct-routing assertions require a capability-capable launcher; separately gating that section on non-root setuid fallback hosts remains a test-portability follow-up