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>

.codex/skills/review-pr/SKILL.md

@@ -104,6 +104,8 @@ Validation:
 - Post review proactively if verdict is clear.
 - Mention validation commands you actually ran.
 - No bracketed author/tool tags.
+- Accept `docs/adr/**` when it records a direct, durable decision tied to the issue scope.
+- Treat `docs/superpowers/**` and other agent execution artifacts as scope creep. When rejecting docs, explain whether the content should be deleted entirely or rewritten as an ADR.
 
 ## What happens next
 - **PASS (APPROVED):** Developer merges the PR.