Replace built-in proxies with tun2socks transparent proxying

Remove the built-in HTTP/SOCKS5 proxy servers and domain allowlist/denylist
system. Instead, use tun2socks with a TUN device inside the network namespace
to transparently route all TCP/UDP traffic through an external SOCKS5 proxy.

This enables truly transparent proxying where any binary (Go, static, etc.)
has its traffic routed through the proxy without needing to respect
HTTP_PROXY/ALL_PROXY environment variables. The external proxy handles its
own filtering.

Key changes:
- NetworkConfig: remove AllowedDomains/DeniedDomains/proxy ports, add ProxyURL
- Delete internal/proxy/, internal/templates/, internal/importer/
- Embed tun2socks binary (downloaded at build time via Makefile)
- Replace LinuxBridge with ProxyBridge (single Unix socket to external proxy)
- Inner script sets up TUN device + tun2socks inside network namespace
- Falls back to env-var proxying when TUN is unavailable
- macOS: best-effort env-var proxying to external SOCKS5 proxy
- CLI: remove --template/import, add --proxy flag
- Feature detection: add ip/tun/tun2socks status to --linux-features

internal/sandbox/benchmark_test.go

@@ -312,9 +312,7 @@ func skipBenchIfSandboxed(b *testing.B) {
 
 func benchConfig(workspace string) *config.Config {
 	return &config.Config{
-		Network: config.NetworkConfig{
-			AllowedDomains: []string{},
-		},
+		Network: config.NetworkConfig{},
 		Filesystem: config.FilesystemConfig{
 			AllowWrite: []string{workspace},
 		},