docs(openspec): backfill missing proposals

openspec/changes/add-multiarch-release-artifacts/tasks.md

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+## 1. Implementation
+- [x] 1.1 Update the release workflow matrix to add Linux arm64 builds and split build steps by architecture.
+- [x] 1.2 Add macOS universal build flow (merge app bundles before generating universal dmg).
+- [x] 1.3 Introduce Windows MSI maker and required dependencies, and generate MSI in the Windows job.
+- [x] 1.4 Extend `postMake` artifact renaming and architecture mapping logic to generate SHA-256 checksum files.
+- [ ] 1.5 Update release attachments and artifact inventory documentation (if needed).
+- [ ] 1.6 Verify CI outputs for Linux arm64, macOS universal, and Windows MSI.