fix: repair Windows installer CI and remove obsolete artifacts

[ashishmax31]

Summary

• isolate Windows PowerShell 5.1 from the PowerShell 7 module path inherited by the Go integration-test process
• add regression coverage that preserves the module path for PowerShell 7 and removes it only for Windows PowerShell
• remove obsolete installer contract tests and sa.yaml
• remove the Branded URLs section from INSTALL.md
• track the design and implementation plan for the CI repair

This is a follow-up to #5, which was merged with a failing Windows installer check.

Verification

• go test ./... -count=1
• go test -race ./... -count=1
• go vet ./...
• sh -n install.sh
• go build ./cmd/stackdome
• actionlint .github/workflows/*.yml