rename Fence to Greywall as GreyHaven sandboxing component

Rebrand the project from Fence to Greywall, the sandboxing layer of the
GreyHaven platform. This updates:

- Go module path to gitea.app.monadical.io/monadical/greywall
- Binary name, CLI help text, and all usage examples
- Config paths (~/.config/greywall/greywall.json), env vars (GREYWALL_*)
- Log prefixes ([greywall:*]), temp file prefixes (greywall-*)
- All documentation, scripts, CI workflows, and example files
- README rewritten with GreyHaven branding and Fence attribution

Directory/file renames: cmd/fence → cmd/greywall, pkg/fence → pkg/greywall,
docs/why-fence.md → docs/why-greywall.md, example JSON files, and banner.

internal/sandbox/macos_test.go

@@ -4,7 +4,7 @@ import (
 	"strings"
 	"testing"
 
-	"github.com/Use-Tusk/fence/internal/config"
+	"gitea.app.monadical.io/monadical/greywall/internal/config"
 )
 
 // TestMacOS_NetworkRestrictionWithProxy verifies that when a proxy URL is set,
@@ -274,14 +274,14 @@ func TestExpandMacOSTmpPaths(t *testing.T) {
 			want:  []string{".", "~/.cache"},
 		},
 		{
-			name:  "mirrors /tmp/fence to /private/tmp/fence",
-			input: []string{".", "/tmp/fence"},
-			want:  []string{".", "/tmp/fence", "/private/tmp/fence"},
+			name:  "mirrors /tmp/greywall to /private/tmp/greywall",
+			input: []string{".", "/tmp/greywall"},
+			want:  []string{".", "/tmp/greywall", "/private/tmp/greywall"},
 		},
 		{
-			name:  "mirrors /private/tmp/fence to /tmp/fence",
-			input: []string{".", "/private/tmp/fence"},
-			want:  []string{".", "/private/tmp/fence", "/tmp/fence"},
+			name:  "mirrors /private/tmp/greywall to /tmp/greywall",
+			input: []string{".", "/private/tmp/greywall"},
+			want:  []string{".", "/private/tmp/greywall", "/tmp/greywall"},
 		},
 		{
 			name:  "mirrors nested subdirectory",
@@ -290,8 +290,8 @@ func TestExpandMacOSTmpPaths(t *testing.T) {
 		},
 		{
 			name:  "no duplicate when mirror already present",
-			input: []string{".", "/tmp/fence", "/private/tmp/fence"},
-			want:  []string{".", "/tmp/fence", "/private/tmp/fence"},
+			input: []string{".", "/tmp/greywall", "/private/tmp/greywall"},
+			want:  []string{".", "/tmp/greywall", "/private/tmp/greywall"},
 		},
 	}