feat(sandbox): add network_via_proxy_only and allow_direct_dns policy fields (#370)

* feat(sandbox): add network_via_proxy_only and allow_direct_dns policy fields

Config surface for network lockdown: network_via_proxy_only confines a
sandboxed package manager's outbound network to the PMG proxy;
allow_direct_dns is its escape hatch re-opening direct DNS. Both follow
the existing pointer-bool inheritance pattern in MergeWithParent. Lint
warns when allow_direct_dns is set without network_via_proxy_only, where
it has no effect.

The fields are declared and inherited but unread; enforcement lands with
the ExecutionContext plumbing and Seatbelt lockdown translation.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PqMU5GNBbQvQct9nxek1VS

* fix(sandbox): deep-copy new pointer flags in profile resolution

expandPolicyPaths re-points the older pointer booleans so callers cannot
corrupt the registry-cached policy; the new NetworkViaProxyOnly and
AllowDirectDNS fields need the same isolation.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01PqMU5GNBbQvQct9nxek1VS

---------

Co-authored-by: Claude <noreply@anthropic.com>

sandbox/lint_test.go

@@ -3,7 +3,9 @@ package sandbox
 import (
 	"testing"
 
+	"github.com/safedep/dry/utils"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 // cleanPolicy returns a minimally-valid policy with no lint issues.
@@ -230,3 +232,51 @@ func TestLintProfile_OrderingErrorsBeforeWarnsBeforeInfo(t *testing.T) {
 		}
 	}
 }
+
+func TestLintProfile_AllowDirectDNSWithoutLockdown(t *testing.T) {
+	tests := []struct {
+		name                string
+		allowDirectDNS      *bool
+		networkViaProxyOnly *bool
+		wantWarn            bool
+	}{
+		{
+			name:           "allow_direct_dns without lockdown warns",
+			allowDirectDNS: utils.PtrTo(true),
+			wantWarn:       true,
+		},
+		{
+			name:                "allow_direct_dns with lockdown is clean",
+			allowDirectDNS:      utils.PtrTo(true),
+			networkViaProxyOnly: utils.PtrTo(true),
+			wantWarn:            false,
+		},
+	}
+
+	for _, tc := range tests {
+		t.Run(tc.name, func(t *testing.T) {
+			policy := cleanPolicy()
+			policy.AllowDirectDNS = tc.allowDirectDNS
+			policy.NetworkViaProxyOnly = tc.networkViaProxyOnly
+
+			var found *LintIssue
+			for _, i := range LintProfile(policy) {
+				if i.Code == "allow-direct-dns-without-lockdown" {
+					issue := i
+					found = &issue
+					break
+				}
+			}
+
+			if !tc.wantWarn {
+				assert.Nil(t, found)
+				return
+			}
+
+			require.NotNil(t, found)
+			assert.Equal(t, LintLevelWarn, found.Level)
+			assert.Equal(t, "allow_direct_dns", found.Field)
+			assert.Equal(t, "allow_direct_dns has no effect unless network_via_proxy_only is true", found.Message)
+		})
+	}
+}