Build and lint the examples in CI

`MetropolisHastings` and `Bandits` become default targets, so that `lake build`
and `lake lint`, and hence CI, build and lint them along with `RandomDo`.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: RemyDegenne

lakefile.toml

@@ -1,5 +1,5 @@
 name = "RandomDo"
-defaultTargets = ["RandomDo"]
+defaultTargets = ["RandomDo", "MetropolisHastings", "Bandits"]
 lintDriver = "batteries/runLinter"
 testDriver = "Test"