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/tun2socks_embed.go

@@ -32,7 +32,7 @@ func extractTun2Socks() (string, error) {
 		return "", fmt.Errorf("tun2socks: embedded binary not found for %s: %w", arch, err)
 	}
 
-	tmpFile, err := os.CreateTemp("", "fence-tun2socks-*")
+	tmpFile, err := os.CreateTemp("", "greywall-tun2socks-*")
 	if err != nil {
 		return "", fmt.Errorf("tun2socks: failed to create temp file: %w", err)
 	}