ci: install worker deps before discord unit tests

[rz1989s]

Root cause

discord-drift has been red on every scheduled run since the worker tests landed (Sep 7, 14, 21 — e.g. run 35596151629). The failure is not Discord drift:

Error [ERR_MODULE_NOT_FOUND]: Cannot find package 'discord-interactions'
imported from discord/worker/src/index.js

The Unit tests step runs node --test from discord/, which also discovers worker/test/. handlers.test.js imports ../src/index.js, which needs the worker's discord-interactions dependency — but the workflow never installs it, so the file dies at load and the job exits 1. The Alert mod-log on drift step then posts a misleading "drift detected" message to the mod log every week.

Fix

Add npm ci in discord/worker before the unit tests. Nothing else changes.

Validation

• Local repro on main: 62 pass / 1 fail (handlers.test.js load failure) — matches CI exactly.
• With this change: 73/73 pass — the 11 worker tests that were crashing at load now actually run.
• workflow_dispatch run on this branch:

Note: once this merges, the next scheduled run (Mondays 06:00 UTC) exercises the real drift check (verify.js + secrets). If that fails, it's genuine drift — the alert step finally means what it says.

[rz1989s]

Validated end-to-end via workflow_dispatch on this branch: run 35809018437 — all steps green. checkout@v7 / setup-node@v7 (from #10 / #16) both ran clean, unit tests 73/73, and the real drift check passed, so there is no actual server drift — the weekly reds were purely the missing install step.