[Agent Substrate] Enforce Source Discussion Criteria Mapping Gate in epic-review (#11328 Part 1)

What is the purpose of this issue?

Implement Stage 2.5 in epic-review to enforce a semantic bridge between a source Discussion and an Epic's Acceptance Criteria.

What is the context?

This is part 1 of 3 to close the "Discussion-graduation enforcement gap" identified in the #11328 audit. The current workflow protects pre-graduation deliberation and Epic AC completion, but lacks a mechanical gate to ensure all upstream graduation criteria are mapped to Epic ACs. The "Map vs. World Atlas" failure mode allows a local Epic to silently drop terrain from its source Discussion.

Acceptance Criteria

• Update .agents/skills/epic-review/SKILL.md (and/or its references/epic-review-workflow.md audit file) to include a new Stage 2.5: Source Discussion Criteria Mapping Gate.
• Add the trigger condition: Fires when an Epic body cites a Discussion origin, Signal Ledger, [GRADUATED_TO_TICKET], or [RESOLVED_TO_AC].
• Require reviewers to fetch the source Discussion body and extract the Graduation Criteria.
• Require the Epic body to contain a mapping table (e.g., Source Criterion | Epic AC/sub | Status).
• Enforce the failure mode: Missing mapping or unexplained deferrals require REVISIONS_REQUESTED before sub-ticket pickup.

Priority

• priority:high
• ai