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).
- CLAUDE.md: Linux host no longer runs pwsh; PS 5.1 constraints scoped to
Windows guests + Windows runner.
- RUNBOOK: add "Phase C — Python equivalents" table; §6-§13 benchmark records
kept as historical provenance.
- TODO §3.7: close $IsWindows shim bug (resolved-by-replacement — bench run
bypasses the shim).
- plans: mark C1-C8 done + docs C9; flag operational cutover/uninstall pending.
Simone