feat: standardize command error handling (Sprint 4)

- Create withCommandErrorHandling utility in bot/lib/commandUtils.ts
- Migrate economy commands: daily, exam, pay, trivia
- Migrate inventory command: use
- Migrate admin/moderation commands: warn, case, cases, clearwarning,
  warnings, note, notes, create_color, listing, webhook, refresh,
  terminal, featureflags, settings, prune
- Add 9 unit tests for the utility
- Update AGENTS.md with new recommended error handling pattern

AGENTS.md

@@ -141,22 +141,36 @@ throw new UserError("You don't have enough coins!");
 throw new SystemError("Database connection failed");
 ```
 
-### Standard Error Pattern
+### Recommended: `withCommandErrorHandling`
+
+Use the `withCommandErrorHandling` utility from `@lib/commandUtils` to standardize
+error handling across all commands. It handles `deferReply`, `UserError` display,
+and unexpected error logging automatically.
 
 ```typescript
-try {
-  const result = await service.method();
-  await interaction.editReply({ embeds: [createSuccessEmbed(result)] });
-} catch (error) {
-  if (error instanceof UserError) {
-    await interaction.editReply({ embeds: [createErrorEmbed(error.message)] });
-  } else {
-    console.error("Unexpected error:", error);
-    await interaction.editReply({
-      embeds: [createErrorEmbed("An unexpected error occurred.")],
-    });
-  }
-}
+import { withCommandErrorHandling } from "@lib/commandUtils";
+
+export const myCommand = createCommand({
+  data: new SlashCommandBuilder()
+    .setName("mycommand")
+    .setDescription("Does something"),
+  execute: async (interaction) => {
+    await withCommandErrorHandling(
+      interaction,
+      async () => {
+        const result = await service.method();
+        await interaction.editReply({ embeds: [createSuccessEmbed(result)] });
+      },
+      { ephemeral: true } // optional: makes the deferred reply ephemeral
+    );
+  },
+});
+```
+
+Options:
+- `ephemeral` — whether `deferReply` should be ephemeral
+- `successMessage` — a simple string to send on success
+- `onSuccess` — a callback invoked with the operation result
 ```
 
 ## Database Patterns
@@ -240,3 +254,4 @@ describe("serviceName", () => {
 | Environment   | `shared/lib/env.ts`    |
 | Embed helpers | `bot/lib/embeds.ts`    |
 | Command utils | `shared/lib/utils.ts`  |
+| Error handler | `bot/lib/commandUtils.ts` |