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_test.go

@@ -134,8 +134,8 @@ func TestGenerateProxyEnvVars(t *testing.T) {
 			name:     "no proxy",
 			proxyURL: "",
 			wantEnvs: []string{
-				"FENCE_SANDBOX=1",
-				"TMPDIR=/tmp/fence",
+				"GREYWALL_SANDBOX=1",
+				"TMPDIR=/tmp/greywall",
 			},
 			dontWant: []string{
 				"HTTP_PROXY=",
@@ -147,7 +147,7 @@ func TestGenerateProxyEnvVars(t *testing.T) {
 			name:     "socks5 proxy",
 			proxyURL: "socks5://localhost:1080",
 			wantEnvs: []string{
-				"FENCE_SANDBOX=1",
+				"GREYWALL_SANDBOX=1",
 				"ALL_PROXY=socks5://localhost:1080",
 				"all_proxy=socks5://localhost:1080",
 				"HTTP_PROXY=socks5://localhost:1080",
@@ -162,7 +162,7 @@ func TestGenerateProxyEnvVars(t *testing.T) {
 			name:     "socks5h proxy",
 			proxyURL: "socks5h://proxy.example.com:1080",
 			wantEnvs: []string{
-				"FENCE_SANDBOX=1",
+				"GREYWALL_SANDBOX=1",
 				"ALL_PROXY=socks5h://proxy.example.com:1080",
 				"HTTP_PROXY=socks5h://proxy.example.com:1080",
 			},