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/0001-imap-thread-resolution.md

@@ -0,0 +1,35 @@
+---
+type: ADR
+id: "0001"
+title: "IMAP thread resolution prefers In-Reply-To and falls back to References"
+status: active
+date: 2026-04-28
+---
+
+## Context
+
+The IMAP ingress flow stores a `thread_id` before dispatching work to OpenClaw. Previously the bridge only used `In-Reply-To` to find a parent task. Some valid email clients preserve thread ancestry through `References` even when `In-Reply-To` is absent. In those cases, the bridge created a new `thread_id`, which broke conversation continuity and downstream session reuse.
+
+This decision needs to be durable because future refactors of mail parsing, dispatch, or storage must preserve the same thread-resolution behavior.
+
+## Decision
+
+The bridge resolves parent thread linkage by trying `In-Reply-To` first. If no parent task is found, it falls back to `References`, scanning message IDs from last to first and using the first match found in the database. If neither header yields a known parent, the message's own `message_id` becomes the new `thread_id`.
+
+## Options considered
+
+- **Use `In-Reply-To` first, then `References` last-to-first** (chosen): preserves existing behavior, matches common email ancestry ordering, and favors the most recent known parent.
+- **Use only `In-Reply-To`**: simpler, but breaks valid threads when clients omit `In-Reply-To`.
+- **Search `References` from first to last**: may attach to an older ancestor even when a more recent known parent exists.
+- **Normalize all thread linkage into a more complex conversation graph**: more flexible, but out of scope for the current bridge behavior.
+
+## Consequences
+
+Thread continuity is improved for replies that rely on `References`.
+Future contributors have an explicit behavioral contract for threading semantics.
+Mail parsing changes must preserve the header precedence and reverse scan behavior unless a new ADR supersedes this one.
+The current approach still depends on existing message IDs being present in the database; if upstream behavior changes significantly, this decision may need reevaluation.
+
+## Advice
+
+If a future change wants different precedence rules or deeper conversation reconstruction, create a new ADR rather than silently changing this behavior.