docs(phase29am): coreplan step-a if/exit implementation instructions

docs/development/current/main/phases/phase-29am/README.md

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+---
+Status: Active
+Scope: CorePlan 実装準備（lowerer/verifier の穴埋め、仕様不変で段階導入）
+Related:
+- docs/development/current/main/design/coreplan-migration-roadmap-ssot.md
+- docs/development/current/main/design/effect-classification-ssot.md
+- docs/development/current/main/design/exitkind-cleanup-effect-contract-ssot.md
+- docs/development/current/main/phases/phase-29ae/README.md
+---
+
+# Phase 29am: CorePlan Step-A implementation (lowerer/verifier)
+
+Goal: CorePlan で“組み立てる”移行を進める前に、`CorePlan` の語彙が単独でも lower/verify できる状態へ近づける。
+
+SSOT 道筋: `docs/development/current/main/design/coreplan-migration-roadmap-ssot.md`
+
+## P0: Implement CorePlan::If / CorePlan::Exit lowering (minimal)
+
+- 指示書: `docs/development/current/main/phases/phase-29am/P0-COREPLAN-LOWERER-IF-EXIT-INSTRUCTIONS.md`
+- ねらい: 既存のルーティング/観測を変えずに、CorePlan 側の “未対応エラー” を減らす（仕様不変）
+- Gate: `./tools/smokes/v2/profiles/integration/joinir/phase29ae_regression_pack_vm.sh`
+