Add expect(transcript).toContainText() for deterministic transcript assertions

[gaojude]

Asserting the agent never said or reached for something currently needs either a judge run (wasteful, and .not.toSatisfyCriterion would invert the judge's fail-closed default into fail-open) or a manual readFileSync(transcriptPath()). This adds a deterministic, judge-free matcher on the transcript sentinel:

expect(transcript).not.toContainText('getServerSideProps');

Misuse (wrong subject, empty needle) and a missing or empty transcript throw instead of returning pass: false, because a returned failure inverts into a silent pass under .not, and an uncaptured transcript is an infra failure rather than evidence of absence. A .not violation cites where the needle appears: [transcript] expected NOT to contain "getServerSideProps", but found it at char 34: …context…. The transcript is the agent's native format (claude-code writes raw session JSONL), so quotes and newlines appear JSON-escaped there; the README says to stick to identifier-like needles.