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/config/config_test.go

@@ -249,10 +249,10 @@ func TestDefaultConfigPath(t *testing.T) {
 	if path == "" {
 		t.Error("DefaultConfigPath() returned empty string")
 	}
-	// Should end with fence.json (either new XDG path or legacy .fence.json)
+	// Should end with greywall.json (either new XDG path or legacy .greywall.json)
 	base := filepath.Base(path)
-	if base != "fence.json" && base != ".fence.json" {
-		t.Errorf("DefaultConfigPath() = %q, expected to end with fence.json or .fence.json", path)
+	if base != "greywall.json" && base != ".greywall.json" {
+		t.Errorf("DefaultConfigPath() = %q, expected to end with greywall.json or .greywall.json", path)
 	}
 }