feat(a3): port build pipeline (vm new, build run, artifacts collect)
Implements Phase A3 of plans/implementation-plan-A-B.md:
- commands/vm.py: vm new (replaces New-BuildVM.ps1)
- commands/build.py: build run (replaces Invoke-RemoteBuild.ps1) with WinRM/SSH dispatch and stdout streaming for act_runner
- commands/artifacts.py: artifacts collect (replaces Get-BuildArtifacts.ps1) using transport.fetch()
- 3 PS scripts reduced to shims preserving \0
- Pester tests/{New-BuildVM,Wait-VMReady,Remove-BuildVM}.Tests.ps1 removed; equivalent cases covered in pytest
- Phase C hook: build/artifacts accept opaque VmHandle/vmx; no Windows path assumptions
Tests: 91 pytest, ruff clean, mypy --strict clean, coverage 78.27% (>=70 gate).
Master checklist + step A3 attivita + 'definizione di fatto' updated; A3-closeout.md added.
Simone
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