chore(tooling): update write-test skill for automatic tx gas-limit

[danceratopz]

🗒️ Description

Since #2969 the transaction gas_limit is optional and auto-fills when omitted, but the write-test skill never mentioned it, so agents kept adding the redundant gas_limit=fork.transaction_gas_limit_cap() boilerplate. This adds a minimal Transactions section: omit gas_limit by default, set it explicitly only for gas-sensitive tests.

🔗 Related Issues or PRs

Follows up on #2969 (automatic transaction gas-limit).

✅ Checklist

• All: Ran fast static checks to avoid unnecessary CI fails, see also Code Standards and Enabling Pre-commit Checks:

  just lint-md
  just spellcheck

• All: PR title adheres to the repo standard - it will be used as the squash commit message and should start type(scope):.
• All: Considered updating the online docs in the ./docs/ directory.
• All: Set appropriate labels for the changes (only maintainers can apply labels).

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (e35d103) to head (453172d).
⚠️ Report is 8 commits behind head on forks/amsterdam.

Additional details and impacted files

@@                  Coverage Diff                  @@
##           forks/amsterdam     #2995       +/-   ##
=====================================================
+ Coverage            81.28%   100.00%   +18.71%     
=====================================================
  Files                  620         1      -619     
  Lines                36641         1    -36640     
  Branches              3311         0     -3311     
=====================================================
- Hits                 29784         1    -29783     
+ Misses                6335         0     -6335     
+ Partials               522         0      -522     

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