New $.process.run capability for plugin hook scripts

Plugin hook scripts can now call $.process.run alongside the existing fs.*, store.*, and http.fetch namespace, letting a hook spawn and run arbitrary external commands rather than only reading files or making HTTP calls. The implementation validates arguments, handles SIGTERM-then-SIGKILL shutdown, caps captured stdout/stderr size, runs under toolCgroupClass: "plugin", and applies default and maximum timeouts.