Review runner: propagate PII abstention (parse_error) into the PII lane

Follow-up from PR #1090 review (Codex P1, comment 3768687908): when the composite check's `checkQueryPii` abstains (`data.pii.parse_error`), the review runner's `detectPii` returns `{ columns: [] }`, indistinguishable from a verified-clean result, and the PII lane treats the model as clean.

Practical exposure is currently nil — an unparseable model already fails the review loudly via the validation/lint lanes, and the abstention is visible in the composite output and fails closed in the CLI/tool paths — but for the "degrade loudly" contract the lane should know the difference.

Scope: extend `ReviewRunner.detectPii` to return an optional `abstained` flag (optional field, non-breaking for mocks), set it from `pii.parse_error` in the dispatcher runner, and have the orchestrator PII lane mark PII checks as unverified (rather than clean) for that file.