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/utils.go

@@ -51,8 +51,8 @@ func NormalizePath(pathPattern string) string {
 // Used on macOS where transparent proxying is not available.
 func GenerateProxyEnvVars(proxyURL string) []string {
 	envVars := []string{
-		"FENCE_SANDBOX=1",
-		"TMPDIR=/tmp/fence",
+		"GREYWALL_SANDBOX=1",
+		"TMPDIR=/tmp/greywall",
 	}
 
 	if proxyURL == "" {