feat(mail): add group management commands

Add gt mail group subcommands:
- gt mail group list - list all groups
- gt mail group show <name> - show group details
- gt mail group create <name> [members...] - create new group
- gt mail group add <name> <member> - add member
- gt mail group remove <name> <member> - remove member
- gt mail group delete <name> - delete group

Includes validation for group names and member patterns.
Supports direct addresses, wildcards, @-patterns, and nested groups.

Part of gt-xfqh1e.7 (group commands task).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/cmd/mail_group_test.go

@@ -0,0 +1,73 @@
+package cmd
+
+import "testing"
+
+func TestIsValidGroupName(t *testing.T) {
+	tests := []struct {
+		name string
+		want bool
+	}{
+		{"ops-team", true},
+		{"all_witnesses", true},
+		{"team123", true},
+		{"A", true},
+		{"abc", true},
+		{"my-cool-group", true},
+
+		// Invalid
+		{"", false},
+		{"with spaces", false},
+		{"with.dots", false},
+		{"@team", false},
+		{"group/name", false},
+		{"team!", false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			if got := isValidGroupName(tt.name); got != tt.want {
+				t.Errorf("isValidGroupName(%q) = %v, want %v", tt.name, got, tt.want)
+			}
+		})
+	}
+}
+
+func TestIsValidMemberPattern(t *testing.T) {
+	tests := []struct {
+		pattern string
+		want    bool
+	}{
+		// Direct addresses
+		{"gastown/crew/max", true},
+		{"mayor/", true},
+		{"deacon/", true},
+		{"gastown/witness", true},
+
+		// Wildcard patterns
+		{"*/witness", true},
+		{"gastown/*", true},
+		{"gastown/crew/*", true},
+
+		// Special patterns
+		{"@town", true},
+		{"@crew", true},
+		{"@witnesses", true},
+		{"@rig/gastown", true},
+
+		// Group names
+		{"ops-team", true},
+		{"all_witnesses", true},
+
+		// Invalid
+		{"", false},
+		{"@", false},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.pattern, func(t *testing.T) {
+			if got := isValidMemberPattern(tt.pattern); got != tt.want {
+				t.Errorf("isValidMemberPattern(%q) = %v, want %v", tt.pattern, got, tt.want)
+			}
+		})
+	}
+}