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/linux_features_stub.go

@@ -15,6 +15,9 @@ type LinuxFeatures struct {
 	HasCapBPF       bool
 	HasCapRoot      bool
 	CanUnshareNet   bool
+	HasIpCommand    bool
+	HasDevNetTun    bool
+	HasTun2Socks    bool
 	KernelMajor     int
 	KernelMinor     int
 }
@@ -39,6 +42,11 @@ func (f *LinuxFeatures) CanUseLandlock() bool {
 	return false
 }
 
+// CanUseTransparentProxy returns false on non-Linux platforms.
+func (f *LinuxFeatures) CanUseTransparentProxy() bool {
+	return false
+}
+
 // MinimumViable returns false on non-Linux platforms.
 func (f *LinuxFeatures) MinimumViable() bool {
 	return false