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.

scripts/release.sh

@@ -150,4 +150,4 @@ git push origin "$NEW_VERSION"
 echo ""
 info "✓ Released $NEW_VERSION"
 info "GitHub Actions will now build and publish the release."
-info "Watch progress at: https://github.com/Use-Tusk/fence/actions"
+info "Watch progress at: https://gitea.app.monadical.io/monadical/greywall/actions"