R1:76194a0dc1dd

Add "Autoplans" to Harbormaster

Summary:
Ref T8095. Two general problems:

- I want Harbormaster to own all lint and unit test results.
- I don't want users to have to configure anything for `arc` to keep working automatically.

These are in conflict because generic lint/unit test ownership in Harbormaster requires that build targets exist which we can attach build results to. However, we can't currently create build targets on demand: Harbormaster assumes it is responsible for creating targets, then…