feature request: bisect script mode

The `--bisect` mode works well if you have something that can be a target, but it would be helpful to have a script mode (analogous to git's [`bisect run`](https://git-scm.com/docs/git-bisect)) for testing things that aren't easy to express as a target.

An example is [here](https://github.com/bazelbuild/bazel/issues/26552) for a bisect that's hard to express as a target.

Start by tracing the existing `--bisect` mode and how it accepts and evaluates a target. Compare the requested behavior with Git's `bisect run` documentation and the linked Bazel example, then determine the command, exit-status, and testing expectations for script mode. Done means a non-target check can drive the bisect reliably.

Written by the indexing model from the issue text.