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/sqlite.go

@@ -389,9 +389,13 @@ func (d *Database) MarkStaleAgentsOffline(olderThan time.Duration) (int, error)
 	return int(n), nil
 }
 
-func (d *Database) DeleteAgent(id string) error {
-	_, err := d.Exec("DELETE FROM agents WHERE id = ?", id)
-	return err
+func (d *Database) DeleteAgent(id string) (bool, error) {
+	res, err := d.Exec("DELETE FROM agents WHERE id = ?", id)
+	if err != nil {
+		return false, err
+	}
+	n, _ := res.RowsAffected()
+	return n > 0, nil
 }
 
 // FindAgentByMAC returns the agent ID for an agent whose MAC address matches.