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/config/config.go

@@ -11,13 +11,14 @@ import (
 )
 
 type Config struct {
-	Shell     ShellConfig     `mapstructure:"shell"`
-	NetBox    NetBoxConfig    `mapstructure:"netbox"`
-	NetBoxes  []NetBoxConfig  `mapstructure:"netboxes"`
-	Resolver  ResolverConfig  `mapstructure:"resolver"`
-	Cache     CacheConfig     `mapstructure:"cache"`
-	SSH       SSHConfig       `mapstructure:"ssh"`
-	Shortcuts ShortcutsConfig `mapstructure:"shortcuts"`
+	Shell      ShellConfig      `mapstructure:"shell"`
+	NetBox     NetBoxConfig     `mapstructure:"netbox"`
+	NetBoxes   []NetBoxConfig   `mapstructure:"netboxes"`
+	Resolver   ResolverConfig   `mapstructure:"resolver"`
+	Cache      CacheConfig      `mapstructure:"cache"`
+	SSH        SSHConfig        `mapstructure:"ssh"`
+	Shortcuts  ShortcutsConfig  `mapstructure:"shortcuts"`
+	Completion CompletionConfig `mapstructure:"completion"`
 }
 
 type ShellConfig struct {
@@ -47,6 +48,11 @@ type SSHConfig struct {
 	DefaultPort int    `mapstructure:"default_port"`
 }
 
+type CompletionConfig struct {
+	// MaxResults limits the number of hostnames offered per completion; 0 = unlimited.
+	MaxResults int `mapstructure:"max_results"`
+}
+
 type ShortcutsConfig struct {
 	Domains      []string `mapstructure:"domains"`
 	StripHyphens bool     `mapstructure:"strip_hyphens"`