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.

examples/01-dev-server/package.json

@@ -1,7 +1,7 @@
 {
   "name": "dev-server-demo",
   "version": "1.0.0",
-  "description": "Demo: Dev server with Redis in fence sandbox",
+  "description": "Demo: Dev server with Redis in greywall sandbox",
   "type": "module",
   "main": "app.js",
   "scripts": {