On Debian 13 (trixie), policykit-1 was retired and pkexec moved
into a standalone package. The previous Depends: policykit-1
made `apt install` fail with \"none of the choices are installable\"
on a fresh trixie VM.
Switch to Depends: pkexec | policykit-1 so apt can satisfy the
constraint on both:
- bookworm: policykit-1 (which provides /usr/bin/pkexec)
- trixie: pkexec (standalone)
Also updates SHA256SUMS and the install README accordingly.
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