fix: signal entire process group on Unix shutdown

[haydenk]

Summary

• Fixes Fix: signal entire process group on terminate/kill (Unix) #7 — signals are now sent to the entire process group (-pgid) so grandchildren (e.g. workers spawned by sh) also receive SIGTERM/SIGKILL on shutdown
• Each child process is placed in its own process group via SysProcAttr{Setpgid: true} so the group kill is scoped correctly
• Falls back to signalling the individual process if the group kill fails
• Also fixes the lint task to correctly locate and invoke staticcheck when it is installed in $GOBIN but not on $PATH

Test plan

• Start a Procfile app where a process spawns child processes (e.g. php-fpm, a shell wrapper)
• Send SIGTERM / Ctrl-C and verify all grandchildren are terminated, not just the direct child
• Run mise run lint in a clean env where staticcheck is not on $PATH and verify it still runs

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