Simplify workflow and refresh MoonBit test blocks

[bobzhang]

Summary

• merge the separate refactoring step into the package-local editing step
• tighten the semantic rename and compatibility guidance
• update four executable examples for the current nightly compiler warnings

CI fix

The nightly job denies warnings and began rejecting an inexhaustive guard, an ambiguous empty-map literal, and deprecated to_repr(...) calls. The examples now use an explicit guard failure branch, Map([]), and Repr(...).

Impact

The workflow is slightly shorter, related editing guidance stays together, and all documented mbt check examples remain valid on the nightly toolchain.

Validation

• moon test moonbit-agent-guide/SKILL.mbt.md --warn-list -1-2-3-6-7-27-28-68 --deny-warn --no-render — 15 passed
• git diff --check
• parsed and verified the skill YAML frontmatter