Add tomswall agent (control-plane pull mode)

Add `tomswall agent`: it pulls this device's compiled config from tomswallapi,
differentially applies it, and reports the applied generation. It caches the
last known-good config and, when the control plane is unreachable, keeps
applying that cache — it never fails closed.

- internal/agent: rendered-config types, HTTP client (fetch + status report),
  on-disk cache, on-device DNS resolver for dns sets (honors the device's
  configured resolver, fail-safe on lookup failure), and the pull-apply-report
  loop behind a mockable Applier.
- Translate the interface-agnostic, address-matched rendered model into native
  tomswall config using the "all:<cidr>" any-interface source/dest form, reusing
  the existing differential engine. Named-set members are inlined as concrete
  addresses (native nft set references are a tracked follow-up).
- cmd/tomswall: wire the `agent` subcommand (flags + TOMSWALL_* env, --once).
- Unit tests: translation, cache, and the don't-fail-closed fallback loop.
- Add DESIGN.md documenting the control-plane architecture.

internal/shorewall/parser_test.go

@@ -187,11 +187,11 @@ SINGLE_QUOTED='another'
 	}
 
 	tests := map[string]string{
-		"IP_FORWARDING":  "Yes",
-		"LOG_LEVEL":      "info",
+		"IP_FORWARDING":   "Yes",
+		"LOG_LEVEL":       "info",
 		"STARTUP_ENABLED": "Yes",
-		"QUOTED_VALUE":   "some value",
-		"SINGLE_QUOTED":  "another",
+		"QUOTED_VALUE":    "some value",
+		"SINGLE_QUOTED":   "another",
 	}
 	for k, want := range tests {
 		got, ok := conf[k]