docs: update moonbit rule — not(x) is deprecated in favor of !expr

[paveg]

Summary

• Update the Boolean negation rule in .claude/rules/moonbit.md to reflect the current MoonBit compiler recommendation (!expr instead of not(x)).
• No code changes — the codebase already uses ! exclusively.

Evidence the rule is stale

• Compiler warning on not(x): Warning: [0023] deprecated: Use !expr instead
• grep -rn '\bnot(' src/ returns zero matches as of today.

Test plan

• Rule file updated
• Verified no remaining not( usages in src/

Closes #15

[codecov-commenter]

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 62.48%. Comparing base (75361f8) to head (a86639b).
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Additional details and impacted files

@@           Coverage Diff           @@
##             main      #16   +/-   ##
=======================================
  Coverage   62.48%   62.48%           
=======================================
  Files          16       16           
  Lines         733      733           
=======================================
  Hits          458      458           
  Misses        275      275           

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