docs: add micro receipt example

[aDragon0707]

Summary / 摘要

This PR adds Micro Receipt guidance for small maintenance tasks.

本 PR 新增 Micro Receipt 指南，用于小型维护任务。

Changes / 改动

• Add examples/micro_receipt_maintenance.yaml.

• Add a Micro Receipt For Small Maintenance Tasks section to RECEIPT.md.

• Link the guidance from English and Chinese README core docs.

• 新增 examples/micro_receipt_maintenance.yaml。

• 在 RECEIPT.md 增加 Micro Receipt For Small Maintenance Tasks 小节。

• 从中英 README 的核心文档列表链接该指南。

Boundary / 边界

• Micro Receipt is not a new protocol type.

• It does not add fields, remove required fields, or change validator behavior.

• It is only the smallest useful way to write a normal SACP receipt for narrow, low-risk maintenance tasks.

• It must not be used to hide risk or skip evidence.

• Micro Receipt 不是新协议类型。

• 不新增字段、不删除必填字段、不改变 validator 行为。

• 它只是给低风险、小范围维护任务使用的最小正常 SACP receipt 写法。

• 不能用它隐藏风险或跳过证据。

Validation / 验证

python validator.py examples/micro_receipt_maintenance.yaml
python validator.py --examples --strict
git diff --check

Result: pass.
结果：通过。