Step 6 of v3.4.1 plan. Adds the validator quartet
(Content/Object/Consistency/CLI) for NEXT-SESSION-PROMPT.local.md
frontmatter (produced_by, produced_at). State-anchored staleness check
is the primary refusal; 24h wall-clock drift downgraded to soft warning
to avoid false positives on weekend pauses.
Internal scaffolding consumed by Step 8 (Phase 1.5 wire-up). User-facing
docs land with Step 14 (CHANGELOG + README + version bump).
Tests 335 -> 346 (+11): 9 unit + 2 CLI shim cases.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
