Prove a task is actually complete before trusting an agent's "done" claim. Runs the given test command and, if the directory is a git repo, diffs the actual changes. Returns PASS only if the tests pass (exit 0) and — when expect_changed is provided — every claimed path actually changed. This is the gate between "agent says done" and "I believe it."

Bug-first discipline: before fixing a bug, confirm you actually reproduced it. Runs a command you expect to FAIL and returns PASS only if it genuinely fails (non-zero exit) — i.e., the bug is reproduced. If the command passes, you haven't reproduced the bug yet and shouldn't start 'fixing' it.