Add fleet registry removal with confirm modal and agent_removed WS.

Operators can remove machines from Crucible and dashboard rosters with honest messaging that deletion is registry-only; bulk select, oath ledger entries, and Go/Vitest coverage included.

server/internal/db/oath_ledger.go

@@ -18,6 +18,7 @@ const (
 	OathSpreadDiscoveredHost = "spread_discovered_host"
 	OathStrainHospice        = "strain_hospice"
 	OathReconScan            = "recon_scan"
+	OathAgentRemoved         = "agent_removed"
 )
 
 // Oath outcomes.