proclive: add process-liveness package

New leaf package that captures a process's identity (PID + start-time fingerprint, plus host/boot guards) and reports whether that exact process is still alive. ResolveOwner walks up the process tree to the first non-shell, non-entire ancestor (the agent that spawned our hook), skipping the Go toolchain too so local-dev's go run wrapper isn't mistaken for the owner. Check returns Alive/Dead/Unknown — Dead on a missing PID, start-time mismatch (PID reuse), or reboot, and Unknown when it can't confirm the host/boot or the platform can't introspect (Windows), so callers fail closed to a timeout rather than trusting a stale PID.

Stdlib + golang.org/x/sys/unix only, so session/strategy/cli can import it without an import cycle.