Remediate review-of-review findings (thoth-l0n, thoth-hai, thoth-hu3, thoth-4or, thoth-0x0, thoth-6rt, thoth-v2j)

[axAilotl]

Summary

Follow-up remediation from the review of the thoth-7v1 fix epic (PR #1 post-merge review). Three parallel streams, merged octopus; full suite green: 410 passed, 2 skipped.

• thoth-l0n — pi skill artifact_types silent default now logs a loud warning naming the skill and resolved scope (fallback kept for compat, no longer silent)
• thoth-hai — map_bounded logs every collected worker error before raising the first
• thoth-hu3 — retention scan logs wiki pages with unreadable frontmatter instead of silently skipping
• thoth-4or — removed dead _validate_status_transition call; collapsed three copy-pasted artifact-review methods into one helper
• thoth-0x0 — semantic memory rejected-candidate evidence check now uses one batched IN query (was N+1)
• thoth-6rt — remaining naive-local datetime.now() writers routed through core/time_utils UTC helpers (ingestion_runtime, research_graph, archivist_state, metadata_db, wiki_scaffold); added naive→UTC coercion at the archivist due-date comparison for legacy persisted state
• thoth-v2j — _first_present_value / _group_by_optional_attr consolidated into new core/collection_utils.py; _WIKI_DOCUMENT_CACHE bounded at 1024 entries with oldest-first eviction

Not included: thoth-13k (evidence-free promotion policy) — a product decision, filed separately for the operator.

Test plan

• PYTHONPATH=. .venv/bin/python -m pytest tests -q -x --ignore=tests/test_archivist_benchmark.py → 410 passed, 2 skipped
• python3 -m compileall -q core collectors processors thoth.py thoth_api.py tests