feat(completion): fast zsh completion for large host caches

Cobra's generated zsh script spawns a subshell per candidate and sends
every host through _describe, so <Tab> hangs for seconds with thousands
of cached hosts. netssh now ships its own script: subcommands and flags
go through _describe, plain hostnames through compadd. 5000 hosts
complete in ~0.08 s instead of ~4 s.

Add completion.max_results to cap the number of offered hostnames and
sort host candidates for a stable order.

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

internal/hook/completion_test.go

@@ -88,7 +88,7 @@ func TestCommandAliasCompletesServerNamesAcrossShells(t *testing.T) {
 			case "bash":
 				err = root.GenBashCompletionV2(&canonical, true)
 			case "zsh":
-				err = root.GenZshCompletion(&canonical)
+				_, err = canonical.WriteString(hook.ZshCompletionScript)
 			case "fish":
 				err = root.GenFishCompletion(&canonical, true)
 			}
@@ -149,7 +149,8 @@ compinit -u -D
 alias j=netssh
 autoload -Uz _j
 # Capture matches without requiring an interactive ZLE completion widget.
-_describe() { print -rl -- "${completions[@]}"; }
+_describe() { print -rl -- "${(P)4}"; }
+compadd() { print -rl -- "$@"; }
 words=(j ser)
 CURRENT=2
 _j
@@ -169,3 +170,38 @@ complete --do-complete 'j ser'
 		})
 	}
 }
+
+func TestZshCompletionSplitsHostsAndCommands(t *testing.T) {
+	zsh, err := exec.LookPath("zsh")
+	if err != nil {
+		t.Skip("zsh unavailable")
+	}
+	dir := t.TempDir()
+	backend := "#!/bin/sh\nprintf 'web01\\nweb:02\\ncache\\tManage the cache\\n:36\\n'\n"
+	if err := os.WriteFile(filepath.Join(dir, "netssh"), []byte(backend), 0o755); err != nil {
+		t.Fatal(err)
+	}
+	if err := os.WriteFile(filepath.Join(dir, "_netssh"), []byte(hook.ZshCompletionScript), 0o644); err != nil {
+		t.Fatal(err)
+	}
+	script := `compdef() { :; }
+source "$DIR/_netssh"
+_describe() { print -r -- "describe ${(P)4}"; }
+compadd() { print -r -- "compadd $*"; }
+_files() { print -r -- files; }
+words=(netssh "")
+CURRENT=2
+_netssh
+`
+	cmd := exec.Command(zsh, "-f", "-c", script)
+	cmd.Env = append(os.Environ(), "DIR="+dir, "PATH="+dir+string(os.PathListSeparator)+os.Getenv("PATH"))
+	out, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("%v\n%s", err, out)
+	}
+	// Directive 36 = NoFileComp|KeepOrder: unsorted host group, no file fallback.
+	want := "describe cache:Manage the cache\ncompadd -V hosts -X %Bhosts%b -- web01 web:02\n"
+	if string(out) != want {
+		t.Fatalf("got:\n%s\nwant:\n%s", out, want)
+	}
+}