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/dangerous.go

@@ -39,14 +39,14 @@ func GetDefaultWritePaths() []string {
 		"/dev/tty",
 		"/dev/dtracehelper",
 		"/dev/autofs_nowait",
-		"/tmp/fence",
-		"/private/tmp/fence",
+		"/tmp/greywall",
+		"/private/tmp/greywall",
 	}
 
 	if home != "" {
 		paths = append(paths,
 			filepath.Join(home, ".npm/_logs"),
-			filepath.Join(home, ".fence/debug"),
+			filepath.Join(home, ".greywall/debug"),
 		)
 	}