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/validate-issue/SKILL.md

@@ -35,6 +35,7 @@ description: Phase 1 skill. Use when a Gitea issue is created or updated and nee
      - Stop. Wait for user to respond, then re-run this skill.
    - **Clear and feasible:**
      - Post Final Requirements (see format below).
+     - If the change likely needs a durable ADR, add a note: "ADR recommended: this change locks in a behavior/contract that future work must preserve."
      - This locks the scope and signals Phase 2 can begin.
 
 ## Final Requirements format