0.3.1 — Revert /usr/sbin path move

Patch release that reverts the /usr/bin → /usr/sbin move from v0.3.0. The reasoning that landed v0.3.0 cited FHS §4.10's letter ("system administration daemons live in sbin"), but the systemd-era convention has moved past that distinction: journalctl, systemctl, loginctl, udevadm, podman, and containerd all ship in /usr/bin despite being administrator commands. Fedora 42+ has formally retired the bin/sbin split — %{_sbindir} == %{_bindir} == /usr/bin — and Debian Trixie has the merge on its roadmap. The v0.3.0 layout was on the wrong side of that trajectory and produced two CI-test bugs we had to fix during the v0.3.0 release run; reverting now is cheaper than keeping the migration as the ecosystem unwinds in the same direction.

Breaking

• Daemon binary back to bindir. Packaged installs land at /usr/bin/pamsignal again (was /usr/sbin/pamsignal for the ~30 minutes v0.3.0 was on the apt/dnf gh-pages repos). Dev installs land at /usr/local/bin/pamsignal. The systemd unit's ExecStart is updated by the same configure_file substitution that landed in v0.3.0, so an apt upgrade / dnf upgrade from v0.3.0 → v0.3.1 atomically removes /usr/sbin/pamsignal, installs /usr/bin/pamsignal, and rewrites ExecStart in lockstep — systemctl daemon-reload is auto-fired by the maintainer scripts. Anyone who scripted against the absolute /usr/sbin/pamsignal path (download count for v0.3.0 was 0 at the time of v0.3.1) needs to revert their scripts to /usr/bin/pamsignal.