Add 'exam-corpus' INIT_MODE: docker-entrypoint.sh case -> main.py --mode
exam-corpus -> run_exam_corpus_mode() -> seed_exam_corpus.load(). Driven by
EXAM_CORPUS_MANIFEST (+ DRY_RUN/FORCE/BOARD/SPEC/USER_SUBSET/FIRST_SWEEP env).
Skips gracefully (success) when no manifest is configured, so it is safe in a
comma list like INIT_MODE=infra,seed,exam-corpus before papers are gathered.
Bucket provisioning stays in infra mode.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
