run: add command package runner support

[joeykchen]

No description provided.

[gemini-code-assist]

Code Review

This pull request implements a command runner feature for the gop run command, allowing projects to define custom runners in their gop.mod files. The changes include logic for resolving project directories, building or installing runner binaries, and executing them. Review feedback suggests propagating errors from xgomod.Load instead of swallowing them and notes that the current implementation only supports the first project defined in a gop.mod file, which could lead to issues in multi-project configurations.

[xgopilot]

Overall: Clean separation across runner.go / runner_binary.go / runner_project.go with solid test coverage. A few items worth addressing: the validateMainPackage call is both a redundant process spawn and a TOCTOU window — go build already rejects non-main packages. lookupModulePackage silently swallows all errors (including permission errors, corrupt go.mod), which could mask real problems. Consider adding runner path validation and documenting the trust model for the new runner directive.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.12%. Comparing base (be09c49) to head (3ab2c34).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2689   +/-   ##
=======================================
  Coverage   94.12%   94.12%           
=======================================
  Files          32       32           
  Lines       10054    10054           
=======================================
  Hits         9463     9463           
  Misses        421      421           
  Partials      170      170           

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[xgopilot]

@joeykchen Thanks for the trigger request.

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