Add task completion receipt guidance

Author: aDragon0707

docs/experimental/task-receipts.md

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+# Task completion receipts
+
+MCP clients and servers can exchange rich task and tool data, but a task result is not always the same thing as an auditable completion state.
+
+For safety-sensitive workflows, consider adding a small receipt to the end of a task or tool-driven workflow. A receipt separates what the agent or tool claims from the evidence that supports the claim, the next owner, and any human approval boundary.
+
+## Why this matters
+
+A final task message such as:
+
+```text
+Done. All tests passed. Ready to publish.
+```
+
+contains multiple operational claims:
+
+- the task is complete
+- tests passed
+- the output is ready for an external action
+
+Those claims may require different evidence, such as tool output, logs, file diffs, test output, trace items, or human approval.
+
+## Minimal receipt shape
+
+```yaml
+status_code: 412
+status_text: missing_evidence
+summary: "The task claimed completion and test success, but did not attach command output."
+claims:
+  - claim: "Task is complete."
+    support_status: unverified
+    evidence: []
+    required_fix: "Attach the relevant tool result, file diff, log, or trace item."
+  - claim: "All tests passed."
+    support_status: unsupported
+    evidence: []
+    required_fix: "Attach the test command and output, or downgrade the claim."
+next_owner: ProducingAgent
+human_decision_required: false
+```
+
+## Safety boundaries
+
+Receipts are useful when a task result may cross a boundary:
+
+- publishing or deploying
+- writing to long-term memory
+- sending messages or comments
+- spending money
+- changing external systems
+- reporting tests or verification as complete
+
+If a boundary requires approval, the receipt should say so explicitly:
+
+```yaml
+human_decision_required: true
+required_fix: "Require approval before publishing."
+```
+
+## Relationship to MCP data
+
+MCP data can provide evidence for a receipt:
+
+```text
+tool result -> evidence item
+task state -> status code
+client/server trace -> evidence path
+approval or user confirmation -> human approval evidence
+```
+
+The receipt does not replace MCP protocol messages. It is a final-state summary that helps humans and downstream systems audit whether a completion claim is supported.
+
+Reference protocol:
+
+```text
+SACP: https://github.com/aDragon0707/sacp
+```