Add ADR-004 documenting the producer trust boundary: untrusted producers (LLM,
agent, human; erroneous and malicious), the trust-but-handle vs verify-and-mitigate
postures, error-locality and quarantine-with-provenance principles, and the concrete
activity-core mechanisms.
Implement producer-agnostic guardrails in executor.py, applied uniformly on the
happy path and the recovery path via _partition_items: structural-type -> schema ->
structural caps (_MAX_DEPTH, _MAX_STRING_LEN) -> reference allow-list -> count cap.
Each quarantine carries a reason. Closes the happy-path maxItems count cap deferred
from T03 (valid 9-item report keeps 7, quarantines 2). Reference allow-list reads
context["known_candidates"] via _allow_list_from_context; inert until a resolver
populates it. SCOPE.md updated (executor bullet + ADR list); no INTENT drift.
New tests: happy-path count cap, oversized-string guardrail, allow-list rejection.
Full suite: 218 passed, 1 skipped.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
