plugin eval can target a non-default eval directory

The plugin eval command accepts a new --eval-dir <dir> flag, and the eval directory can also come from a plugin manifest's experimental.evals key; evals/ remains the fallback, and mis-shaped manifest values produce warnings that name the fix. Case discovery now screens the target path for trust before walking it and resolves it relative to the working directory. The error text shown when ablation runs with no plugin was rewritten to suggest targeting the plugin directory itself.