feat: add Discord help command for servers and DMs

src/discord/commands.ts

@@ -1,4 +1,5 @@
 // Global commands are required for bot DMs; guild-only registrations cannot run there.
 export const commands = [
+  { name: 'help', description: 'Show commands and how to use Minabot' },
   { name: 'today', description: 'View and complete today’s habits' },
 ].map(command => ({ ...command, type: 1, integration_types: [0], contexts: [0, 1] }));