fix(verifier): adapter-raise errored verify checkpoints retry on resume

[gadievron]

Summary

The verify resume classifier _cp_is_error returned True only when the checkpoint had no verification dict or when verification["correct_finding"] == "error" — a value the normal verify error path never writes. That path (an adapter raise — the retryable class) writes the error string on the result dict (finding_verifier.py:800), which the checkpoint writer did not copy, and sets verification["incomplete"] = True. Every errored unit therefore looked like finished work on resume and was adopted, never retried (#286: the reporter's 223/223 adoption with 48 hard errors).

The fix is surgical (per adversarial review): the checkpoint writer — both sequential and parallel — now copies the adapter-raise error string into cp_data["error"], and the classifier retries only checkpoints carrying it (plus the legacy correct_finding == "error" and the missing-verification shapes). verification["incomplete"] alone is deliberately not a retry signal: five non-error fail-safe writers set it (deterministic outcomes — max iterations, truncated finish, no tool calls, degenerate finish, no-agree finish); retrying those on every resume is unbounded waste, and #212 established that refusals are deterministic — an incomplete-based classifier would retry them forever.

Test plan

5 hermetic tests: the adapter-error shape retries; deterministic incomplete does NOT; the completed shape, the missing shapes, and the legacy correct_finding shape unchanged. The classifier is extracted from the live source via inspect — no copy-paste predicate.

Verification evidence (commands + results)

check	command	result
new tests	pytest tests/test_issue286_errored_checkpoint_retry.py -q	5 passed (offline)
hermeticity oracle	env -i HOME=/tmp/fakehome-noconfig python -m pytest tests/test_issue286_errored_checkpoint_retry.py	5 passed
mutation smoke	revert the error-check	the core test fails (restored → green)
full suite	pytest tests/ -q	3124 passed, 2 failed (pre-existing zig local-env, stash-replay verified), 32 skipped
lint	ruff check .	clean

Adversarial loop: 4-seat wave → the over-retry catch (my first fix classified ALL incomplete as errored — the wave identified five non-error incomplete=True writers and the deterministic-refusal infinite-retry risk; fixed with the surgical error-string discriminator copied into the checkpoint by the writer).

Fixes #286