Gen 2 migration Subtask 3: Decommission Gen 1, resume paused schema epic

Story Points: 2

# How?

Part of #292 (Amplify Gen 1 → Gen 2 Rebuild). Full detail in `.claude/plans/can-you-make-a-shimmying-crane.md`, Phase 6. Depends on the Phase 5 sub-issue (alpha cutover) completing its full bake period with the canary workflow green.

1. Remove `amplify/backend/*` (Gen 1 CloudFormation tree) and any now-unused Gen 1 CI steps.
2. Rename `amplify-gen2/` → `amplify/` (single atomic rename — this was the reason Gen 2 was built in parallel at a temporary path rather than in-place, to keep Gen 1 fully intact and deployable as a rollback path for the entire migration window).
3. Update any remaining path references (`backend-build.js`/`backend-build-gen2.js` split, docs, CLAUDE.md's description of the `amplify/` vs `amplify-gen2/` tree) to reflect the new single tree.

**Once this lands, resume the paused work**: the backend schema epic (#244 and subtasks #245/#247/#248, plus #258/#52/#51/#213) and the new-frontend plan, both paused pending this rebuild since it deliberately kept `ts-code/src` (business logic) untouched while the surrounding infra changed shape entirely.

閱讀 `.claude/plans/can-you-make-a-shimmying-crane.md` 中的遷移計畫，重點關注 Phase 6。了解目前 `amplify/backend/` 和 `amplify-gen2/` 的目錄結構。檢查 `backend-build.js` 和 `backend-build-gen2.js` 中的路徑參照。這項工作包括刪除舊的 Gen 1 目錄、重新命名 Gen 2 目錄，以及更新所有相關的建置腳本和文件。開始前確認 canary workflow 為綠色。

由索引模型根據 Issue 內容生成。