security: gate /help, signature modal submit, and cancel_delete_tag on staff role

Closes the remaining non-broccolini interaction paths after the prior
TICKET_BUTTON_HANDLERS gate. After this commit, every bot interaction is
staff-only except the panel buttons (open_ticket / open_ticket_thread /
open_ticket_channel) and their ticket-creation modal submit — those have
to stay public because they're how members and customers open tickets.

Specific changes:

- handlers/commands/index.js: handleCommand no longer has the
  `!== 'help'` carve-out. /help now goes through requireStaffRole like
  every other slash command. Non-staff get the same ephemeral
  "only available to the support team" reply.

- broccolini-discord.js: the signature_modal_* modal-submit handler now
  calls requireStaffRole before writing to StaffSignature. /signature
  already gates the modal display via the slash-command staff check;
  this is defense in depth against directly crafted submissions.

- handlers/buttons.js: cancel_delete_tag moved out of
  FREE_BUTTON_HANDLERS and gated alongside confirm_delete_tag::*. The
  dialog is only shown ephemerally to the staff who triggered
  /response delete, so non-staff can't reach it in normal flow; gating
  keeps the button surface consistent.

Kept public (by design — these are the customer entry points):
  open_ticket / open_ticket_thread / open_ticket_channel buttons
  ticket_modal / ticket_modal_thread / ticket_modal_channel submits

broccolini-discord.js

@@ -11,6 +11,7 @@ const { mongoose } = require('./db-connection');
 // Handlers
 const { handleButton, handleTicketModal } = require('./handlers/buttons');
 const { handleCommand, handleContextMenu, handleAutocomplete } = require('./handlers/commands');
+const { requireStaffRole } = require('./handlers/commands/helpers');
 const { handleDiscordReply } = require('./handlers/messages');
 
 // Services & jobs
@@ -110,6 +111,9 @@ client.on('interactionCreate', async interaction => {
   }
 
   if (interaction.isModalSubmit() && interaction.customId.startsWith('signature_modal_')) {
+    // Staff-only: /signature shows this modal, which is gated; double-gate the
+    // submit path in case an attacker crafts the submission directly.
+    if (await requireStaffRole(interaction)) return;
     // Handle signature modal submit
     try {
       const valediction = interaction.fields.getTextInputValue('valediction');