docs: define approved-plan continuity receipt storage

state/approved-plan-continuity/README.md

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+# Approved Plan Continuity Receipt Storage
+
+This directory stores file-backed continuity receipts for approved-plan flows.
+
+## Scope
+
+This storage definition is intentionally minimal.
+It defines only the receipt location, minimum receipt shape, and filename convention for continuity receipts.
+It does **not** implement receipt writing, hook integration, dispatch orchestration, or gate evaluation logic.
+
+## Receipt file format
+
+- Format: JSON
+- Encoding: UTF-8
+- One receipt per file
+
+## Minimum receipt shape
+
+Each continuity receipt file must contain a JSON object with at least these fields:
+
+- `planId`
+- `currentTask`
+- `nextDerivedAction`
+- `dispatchedAt`
+- `dispatchRunId`
+- `childSessionKey`
+- `replyClosureState`
+
+### Minimal example
+
+```json
+{
+  "planId": "plan_2026_04_24_example",
+  "currentTask": "Task 15",
+  "nextDerivedAction": "dispatch next approved-plan task",
+  "dispatchedAt": "2026-04-24T12:00:00.000+08:00",
+  "dispatchRunId": "dispatch_2026_04_24_example",
+  "childSessionKey": "agent:engineering:subtask-example",
+  "replyClosureState": "open"
+}
+```
+
+## Filename convention
+
+Continuity receipt filenames must follow this pattern:
+
+```text
+receipt-<planId>-<dispatchRunId>.json
+```
+
+## Naming rules
+
+- `<planId>` should match the receipt `planId`
+- `<dispatchRunId>` should match the receipt `dispatchRunId`
+- Use lowercase kebab-case or other filesystem-safe identifiers
+- Do not reuse one file for multiple dispatch runs
+
+## State interpretation
+
+- A receipt in this directory represents a persisted continuity dispatch record for one approved-plan dispatch run.
+- `replyClosureState` is stored alongside the dispatch linkage so later tasks can distinguish an active dispatch record from an allowed non-dispatch closure state.
+- Legal non-dispatch closure values are defined by the plan/runbook logic outside this storage README.