Gmail folder auto-advance + /forward command

- Reply-cycle auto-advance: staff reply files the thread to "Awaiting Reply",
  a customer response files it to "Needs Response" (new GMAIL_LABEL_AWAITING_REPLY
  / GMAIL_LABEL_NEEDS_RESPONSE labels + autoAdvanceFolder, which only moves
  threads still in the auto-cycle and leaves hand-filed folders alone)
- /forward: forward a ticket's email to another address (handlers/commands/forward.js
  + forward composition in services/gmail.js)
- Tests for the auto-advance cycle; label fixtures updated for the new labels

commands/register.js

@@ -403,6 +403,26 @@ async function registerCommands() {
           )
       ),
 
+    new SlashCommandBuilder()
+      .setName('forward')
+      .setDescription("Forward this ticket's email thread to another address")
+      .setContexts([InteractionContextType.Guild])
+      .setIntegrationTypes([ApplicationIntegrationType.GuildInstall])
+      .setDefaultMemberPermissions(PermissionFlagsBits.ManageMessages)
+      .addStringOption(opt =>
+        opt
+          .setName('email')
+          .setDescription('Destination email address')
+          .setRequired(true)
+      )
+      .addStringOption(opt =>
+        opt
+          .setName('note')
+          .setDescription('Optional message to include at the top of the forward')
+          .setMaxLength(1000)
+          .setRequired(false)
+      ),
+
     new SlashCommandBuilder()
       .setName('cancel-close')
       .setDescription('Cancel a pending force-close countdown')