Using `engine_repo_tools`, provide a default `--target-variant` for the `clang_tidy` tool.
Mostly test code added below, the major change is that `--target-variant` now has a default other than `host_debug` if we're running inside of an engine root (which we ~always are). I didn't make any other changes (i.e. to the pre-commit hook) to keep this change smallish.
Also rewrote the `README.md` to represent the current state as best I could.
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