feat: hostname resolver strategy and static hosts

Add the hostname resolver strategy. It connects by the NetBox name
instead of an IP, with resolver.hostname_domain appended to names
without a dot. Its position in the chain decides whether it is always
used or only as a fallback (e.g. [primary_ip, hostname]); names that
are not valid DNS hostnames fall through to the next strategy.

Add static hosts under hosts: for machines that are not in NetBox,
with address, user, port, tags, and their own SSH options. They are
checked before NetBox (by name or shortcut), never expire, survive
cache refresh and clear, and work without any NetBox configured. The
cache overlays them after every load and stores only their last use,
so they appear in completion, aliases, the TUI and its recent list.
SSH options merge global, static host, then rules; rules can match
kind: static. Manage them with netssh hosts list|add|remove and in
netssh configure, which can now also continue without NetBox.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/setup/instances.go

@@ -20,6 +20,18 @@ func configureInstances(cfg *config.Config) ([]config.NetBoxConfig, error) {
 	}
 	for {
 		if len(sources) == 0 {
+			choice := "add"
+			err := huh.NewForm(huh.NewGroup(huh.NewSelect[string]().
+				Title("NetBox").
+				Description("netssh looks up hosts in NetBox. Without NetBox, only static hosts (configured next) are available.").
+				Options(huh.NewOption("Add a NetBox instance", "add"), huh.NewOption("Continue without NetBox (static hosts only)", "none")).
+				Value(&choice))).Run()
+			if err != nil {
+				return nil, err
+			}
+			if choice == "none" {
+				return nil, nil
+			}
 			source, err := editInstance(config.NetBoxConfig{Name: "default"}, sources, "")
 			if err != nil {
 				return nil, err