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.

.github/workflows/release.yml

@@ -75,7 +75,7 @@ jobs:
           {
             "version": "${{ github.ref_name }}",
             "published_at": "$(date -u +%Y-%m-%dT%H:%M:%SZ)",
-            "url": "https://github.com/Use-Tusk/fence/releases/tag/${{ github.ref_name }}"
+            "url": "https://gitea.app.monadical.io/monadical/greywall/releases/tag/${{ github.ref_name }}"
           }
           EOF