Gate: run a real Age of Umbra session on it

The phase gate. Not a code task — a real session, run through Orrey, before phase 2 starts.

- [ ] Create the next Age of Umbra session by hand (seed or one-off script).
- [ ] Confirm the Discord scheduled event, the attendance post and the Google event all appear and agree.
- [ ] Have the roster respond via the buttons; confirm the post rewrites correctly under concurrent clicks.
- [ ] Note here anything that felt wrong on the day — this list feeds phase 2 and 3 scope.

No file or test entry point is named. Start by creating the next Age of Umbra session by hand or with a one-off script through Orrey, then verify the Discord event, attendance post, Google event, and concurrent button responses agree. Done means the checklist is confirmed and any problems are recorded here for later phases.

索引モデルが issue の本文から書いたものです。