docs(spec): add Electron desktop pivot design spec

Captures the agreed architecture for converting game-server-deploy from
a Nest+React web app into a single-binary cross-platform Electron desktop
app. Locked decisions: full pivot to desktop, IPC bridge (Pattern B, no
HTTP listener), local terraform CLI + remote S3 backend, cloud-provider
seams in @gsd/shared with AWS-only impl in v1, Discord receiver
abstracted (AWS Lambda concrete), three-target unsigned MVP. Migration
plan repurposes epics #80-82, closes #83 + stale #5/#6/#7/#10/#43/#103,
and decomposes work into seven new epics across four releasable
milestones.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: CoderCoco