feat(shortcuts): regex rewrite rules for host shortcuts

shortcuts.rewrites applies regex replacements after the domain suffix
is stripped, e.g. ^([a-z]+)[0-9]+- -> $1 turns fsn1-bx1.hoster.de into
fsnbx1. The shortcut works for connecting (cached hosts), shell
aliases, and Tab completion, where it is offered when the full name
does not match the typed text. Rules are editable in netssh configure,
and netssh alias --test HOSTNAME shows a name's shortcut and the cached
hosts it matches.

The shortcut lookup only ran when domains or strip_hyphens were set;
it now also runs for rewrite-only configurations.

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

internal/config/config.go

@@ -57,8 +57,34 @@ type CompletionConfig struct {
 }
 
 type ShortcutsConfig struct {
-	Domains      []string `mapstructure:"domains"`
-	StripHyphens bool     `mapstructure:"strip_hyphens"`
+	Domains      []string          `mapstructure:"domains"`
+	StripHyphens bool              `mapstructure:"strip_hyphens"`
+	Rewrites     []ShortcutRewrite `mapstructure:"rewrites"`
+}
+
+// ShortcutRewrite replaces every match of Pattern (Go regexp syntax) with
+// Replace, which may reference groups as $1. Applied after domain stripping.
+type ShortcutRewrite struct {
+	Pattern string `mapstructure:"pattern"`
+	Replace string `mapstructure:"replace"`
+}
+
+// Enabled reports whether any shortcut normalization is configured.
+func (s ShortcutsConfig) Enabled() bool {
+	return len(s.Domains) > 0 || s.StripHyphens || len(s.Rewrites) > 0
+}
+
+// Validate rejects rewrite rules that cannot be compiled.
+func (s ShortcutsConfig) Validate() error {
+	for i, rewrite := range s.Rewrites {
+		if rewrite.Pattern == "" {
+			return fmt.Errorf("shortcuts.rewrites[%d]: pattern is required", i)
+		}
+		if _, err := regexp.Compile(rewrite.Pattern); err != nil {
+			return fmt.Errorf("shortcuts.rewrites[%d]: invalid pattern %q: %v", i, rewrite.Pattern, err)
+		}
+	}
+	return nil
 }
 
 // Path returns the canonical config file path.
@@ -128,6 +154,9 @@ func Load() (*Config, error) {
 	if err := cfg.ValidateSSH(); err != nil {
 		return nil, err
 	}
+	if err := cfg.Shortcuts.Validate(); err != nil {
+		return nil, err
+	}
 	for i := range cfg.NetBoxes {
 		source := &cfg.NetBoxes[i]
 		source.Name = strings.ToLower(source.Name)