Add exec commands for running arbitrary executables with local bins on PATH

Add exec commands for running arbitrary executables with local bins on PATH: a task in MiMo-V2.6-RL-harbor-code: MiMo-V2.6-RL Code (Harbor) (Harbor dataset). pyarn can already run package scripts, but there's no way to invoke an arbitrary executable (for example a binary installed by a dependency)…

The task

`pyarn` can already `run` package scripts, but there's no way to invoke an arbitrary executable (for example a binary installed by a dependency) with the project's locally installed tools available on `PATH`. Add a family of `exec` commands that do this.

Part of FineEnvs/MiMo-V2.6-RL-harbor-code.