docs: add ADR workflow and update skills

- Add docs/adr/README.md with ADR format, rules, and index
- Add ADR-0001: IMAP thread resolution via In-Reply-To + References
- Add Documentation Rules section to AGENT.md
- Update validate-issue skill: flag ADR-requiring changes
- Rename do-task → implement-task for clearer semantics
- Update review-pr skill: accept docs/adr/, reject agent artifacts

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

AGENT.md

@@ -118,7 +118,16 @@ Validation:
 
 ---
 
-## 7. Comment Style
+## 7. Documentation Rules
+
+- Durable architecture or behavior decisions belong in `docs/adr/**`.
+- `docs/superpowers/**` is for agent execution artifacts and must not be merged as part of product implementation PRs.
+- If a PR needs a lasting technical rationale, author an ADR in `docs/adr/` instead.
+- Reviewers should treat temporary plan/spec docs as scope creep, but accept ADRs that directly document the accepted behavior of the change.
+
+---
+
+## 8. Comment Style
 
 - Use readable multiline markdown.
 - Use short sections (Summary, Changes, Test Plan).