Fix zombie tunnel process on forceful sst dev crash

[vimtor]

closes #6297

when the parent sst process is force-killed, the sudo tunnel subprocess survives and blocks future tunnels

fix: pipe stdin from parent to child. when the parent dies, the kernel closes the pipe, the child detects eof and cleans up

proof of it working

before

time=2026-02-20T16:32:07.982+01:00 level=INFO msg="killing process" pid=88579
time=2026-02-20T16:32:07.986+01:00 level=INFO msg="process killed with term" pid=88579
time=2026-02-20T16:32:07.986+01:00 level=INFO msg="untracked process" pid=88579
time=2026-02-20T16:32:07.986+01:00 level=INFO msg="killing process" pid=88579
time=2026-02-20T16:32:07.986+01:00 level=ERROR msg="failed to send sigterm" pid=88579

after