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/benchmark_test.go

@@ -10,7 +10,7 @@ import (
 	"testing"
 	"time"
 
-	"github.com/Use-Tusk/fence/internal/config"
+	"gitea.app.monadical.io/monadical/greywall/internal/config"
 )
 
 // ============================================================================
@@ -305,8 +305,8 @@ func BenchmarkOverhead(b *testing.B) {
 
 func skipBenchIfSandboxed(b *testing.B) {
 	b.Helper()
-	if os.Getenv("FENCE_SANDBOX") == "1" {
-		b.Skip("already running inside Fence sandbox")
+	if os.Getenv("GREYWALL_SANDBOX") == "1" {
+		b.Skip("already running inside Greywall sandbox")
 	}
 }