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>

docs/adr/README.md

@@ -0,0 +1,75 @@
+# Architecture Decision Records
+
+This directory contains Architecture Decision Records (ADRs) for durable technical decisions in this repository.
+
+## Purpose
+
+ADRs capture cross-cutting or behavior-defining decisions that future contributors need to preserve.
+
+Use an ADR when a change introduces or locks in a decision such as:
+
+- runtime behavior that future work must preserve
+- repository-wide technical policy
+- architecture or integration contracts
+- storage or threading semantics
+- quality gate or workflow policy
+
+Do not use ADRs for:
+
+- temporary implementation plans
+- agent execution checklists
+- one-off debugging notes
+- PR-local task breakdowns
+
+## Format
+
+Each ADR is a markdown file with YAML frontmatter.
+
+```md
+---
+type: ADR
+id: "NNNN"
+title: "Short decision title"
+status: active
+date: YYYY-MM-DD
+---
+
+## Context
+
+Why does this decision need to exist now?
+
+## Decision
+
+**State the decision clearly in one or two sentences.**
+
+## Options considered
+
+- **Option A** (chosen): description with pros and cons
+- **Option B**: description with pros and cons
+- **Option C**: description with pros and cons
+
+## Consequences
+
+What becomes easier?
+What becomes harder?
+What risks were accepted?
+What would trigger re-evaluation?
+
+## Advice
+
+Optional. Capture important external input or review notes.
+```
+
+## Rules
+
+- One decision per file.
+- Name files `NNNN-short-kebab-title.md`.
+- Once active, do not rewrite an ADR's meaning later. Supersede it with a new ADR instead.
+- Keep the index below aligned with the current files in this directory.
+- Do not keep placeholder or mock entries in the index.
+
+## Index
+
+| ID | Title | Status |
+|----|-------|--------|
+| 0001 | IMAP thread resolution prefers In-Reply-To and falls back to References | active |