fix: preserve past Discord events across automation runs

[Copilot]

The fetch-event-node.yml workflow checked out the default branch (which has no scheduled_events.json) before running the script. Since the Discord API only returns active/upcoming events, the merge in get_events.js always started from an empty baseline — discarding all previously accumulated past events on every run.

Changes

• .github/workflows/fetch-event-node.yml: Added a step before the script runs that fetches scheduled_events.json from the event-data branch into the workspace, so the existing merge logic can accumulate events correctly over time:

- name: Restore existing events
  run: |
    git fetch origin event-data || true
    git checkout origin/event-data -- scheduled_events.json || true

The || true guards handle the first-run case where the event-data branch or file doesn't yet exist. No changes to get_events.js were needed — its merge logic was already correct.