Promote safe MoonBit script automation

[bobzhang]

Summary

• simplify the MoonBit task checklist and remove redundant workflow guidance
• promote .mbtx as the exclusive format for agent-authored automation
• document effect-appropriate main entry points and default Wasm execution
• use moonbitlang/async/shell for shell-free process orchestration
• stream structured diagnostics with Cmd::each_line and a narrow process policy
• clarify that spawned children require separate host sandboxing

Verification

• ran all five documented .mbtx examples with warnings denied on the default Wasm target
• verified Cmd::each_line under the documented deny-by-default process policy
• verified both successful streaming and nonzero child-exit propagation
• ran git diff --check over the complete PR diff