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.

.golangci.yml

@@ -7,11 +7,11 @@ linters-settings:
     sections:
       - standard
       - default
-      - prefix(github.com/Use-Tusk/fence)
+      - prefix(gitea.app.monadical.io/monadical/greywall)
   gofmt:
     simplify: true
   goimports:
-    local-prefixes: github.com/Use-Tusk/fence
+    local-prefixes: gitea.app.monadical.io/monadical/greywall
   gocritic:
     disabled-checks:
       - singleCaseSwitch