Install pnpm under act on Linux

[virzak]

Under act, the Linux image has no pnpm, so skipping the install left pnpm install failing with command not found. Now only a Windows job under act skips it, since that runs on the host.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.27%. Comparing base (159d40d) to head (e14d5d7).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #170   +/-   ##
=======================================
  Coverage   92.27%   92.27%           
=======================================
  Files          15       15           
  Lines        1708     1708           
  Branches      391      391           
=======================================
  Hits         1576     1576           
  Misses         46       46           
  Partials       86       86           

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