doctor: warn-only on port 5015 collisions
`port_5015_free` is now an advisory check that returns status="warn"
on every failure path instead of blocker/fail, so `make doctor` and
`make install-service` no longer hard-fail on hosts where solstone is
already running at an install path doctor cannot recognize as
self-owned. The check still surfaces the colliding pid and exe path;
it just stops gating downstream make targets.