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.

internal/sandbox/linux_landlock_stub.go

@@ -2,7 +2,7 @@
 
 package sandbox
 
-import "github.com/Use-Tusk/fence/internal/config"
+import "gitea.app.monadical.io/monadical/greywall/internal/config"
 
 // ApplyLandlockFromConfig is a no-op on non-Linux platforms.
 func ApplyLandlockFromConfig(cfg *config.Config, cwd string, socketPaths []string, debug bool) error {