The shared composite action ran 3 steps under `shell: pwsh`, making every
CI job depend on PowerShell on the Linux host. Rewrote them in bash and
dropped the dead $IsLinux / F:\CI branches (Phase B left a single Linux
host serving both windows-build and linux-build runners).
Closes Phase C / idea-3: no CI job or systemd timer requires pwsh on the
Linux host anymore. Ticked all C1-C8 subtask checkboxes and set the plans
to "C chiusa". pwsh is deliberately left installed (operator choice, not a
technical blocker).
Simone
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