Add PowerShell initial state contract

[Aaronontheweb]

Summary

• add the source-compatible PwshInitialStateMode API with safe default Unknown
• require an explicit constrained noninteractive, no-profile startup/environment/module baseline before finite foreach binding proofs
• reset the assertion across decoded child hosts while preserving exact or dynamic invocation cwd
• expand the PowerShell v0.3 design corpus with case-specific state modes and approval-sensitive loop scenarios
• document the consumer contract and update the OpenSpec/implementation plan

Validation

• dotnet build -c Release --no-restore
• dotnet test -c Release --no-build --no-restore (1,742 passed)
• dotnet format --verify-no-changes --no-restore
• pwsh ./scripts/Add-FileHeaders.ps1 -Verify
• openspec validate v0-3-structured-shell-analysis --strict
• design-corpus PII audit
• targeted Slopwatch: zero findings
• adversarial exact-staged review: GO for 1cccd51ee136b8046693d7adfdb7aa2020701fa306465baeb517af9b7f5c368c