docs: add ADR-0002, update flow and add BLOCKLIST_EMAILS

- ADR-0002: external rules pipeline architecture
- README: updated flow to show safety checks → rules → dispatch
- .env.example: added BLOCKLIST_EMAILS

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

docs/adr/0002-external-rules-pipeline.md

@@ -0,0 +1,30 @@
+---
+type: ADR
+id: "0002"
+title: "Ingress uses a configurable external rules pipeline before dispatch"
+status: active
+date: 2026-04-28
+---
+
+## Context
+
+The IMAP ingress flow previously hardcoded whitelist and idempotency checks directly in processMessage. There was no extension point for operator-defined policy such as sender blocking or context-based routing. The flow mixed internal safety rules (anti-loop) with configurable policy (whitelist).
+
+## Decision
+
+Separate internal safety checks (anti-loop, idempotency) from external configurable rules. External rules run in a pipeline after safety checks. Each rule implements a `Rule` interface that evaluates `EmailContext` and returns `RuleResult` (accept/reject + metadata). Rule metadata is persisted as `DispatchContext` on the Task and passed to OpenClaw dispatch.
+
+Pipeline order: anti-loop (internal) → idempotency (internal) → external rules → threading → save → dispatch.
+
+## Options considered
+
+- **Pipeline with Rule interface** (chosen): extensible, testable, each rule is isolated.
+- **Keep everything in processMessage**: simpler but not configurable or testable.
+- **Plugin-based rules via config files**: more flexible but over-engineered for current needs.
+
+## Consequences
+
+New rules can be added by implementing the Rule interface and registering in config.
+Whitelist and blocklist are now env-configurable and independently testable.
+Future rules (domain routing, context selection) fit the same pipeline.
+Task model has a new DispatchContext column (auto-migrated).

docs/adr/README.md

@@ -73,3 +73,4 @@ Optional. Capture important external input or review notes.
 | ID | Title | Status |
 |----|-------|--------|
 | 0001 | IMAP thread resolution prefers In-Reply-To and falls back to References | active |
+| 0002 | Ingress uses a configurable external rules pipeline before dispatch | active |