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.

.gitignore

@@ -1,7 +1,7 @@
-# Binary (only at root, not cmd/fence or pkg/fence)
-/fence
-/fence_*
-/fence-*
+# Binary (only at root, not cmd/greywall or pkg/greywall)
+/greywall
+/greywall_*
+/greywall-*
 
 # Tar archives
 *.tar.gz