Docs: add task completion receipt guidance

[aDragon0707]

Summary

Adds experimental documentation for task completion receipts.

The new page explains how a task/tool-driven workflow can attach a final-state receipt that separates:

• task claims
• supporting evidence
• unsupported claims
• next owner
• human approval boundaries

It also adds the page to the Experimental Tasks docs navigation.

Motivation

MCP data can provide strong evidence, but a final task message such as Done. All tests passed. Ready to publish. is not itself proof that the task is complete or safe to act on.

A small receipt helps humans and downstream systems audit whether completion claims are supported by tool results, logs, traces, or approvals.

Notes

• Docs-only change.
• No SDK behavior change.
• No runtime dependency.
• Safety-oriented guidance for final task state review.
• Mentions SACP only as a reference protocol for the receipt shape.

[aDragon0707]

Note for maintainers: the third-party wallet comment above is unrelated to this PR and was not requested by me. Please ignore it.

I also noticed the remaining failed CI job appears unrelated to this docs-only change: checks / test (3.12, lowest-direct, ubuntu-latest) failed with address already in use during tests/shared/test_streamable_http.py::test_accept_header_wildcard[*/*;q=0.8]. Happy to re-trigger CI from the branch if needed.