chore: Use golangci-lint-action; remove newreposecretwithlibsodium

[alexandear]

• Update GitHub Actions linter workflow to use official golangci/golangci-lint-action instead of ./script/lint.sh. It automatically builds custom golangci-lint cache binaries with ~/.cache/golangci-lint, and runs golangci-lint for each module.
• Remove the libsodium-based example because it requires custom library installation, and golangci-lint fails due to this. I think newreposecretwithxcrypto should be enough.

The cached lint step takes only 40 seconds:

Closes #4118
Updates #4070

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.71%. Comparing base (a12b8d7) to head (cb6e035).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4119   +/-   ##
=======================================
  Coverage   93.71%   93.71%           
=======================================
  Files         211      211           
  Lines       19614    19614           
=======================================
  Hits        18382    18382           
  Misses       1034     1034           
  Partials      198      198           

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[gmlewis]

Does this mean that is it going to check the openapi YAML file every time? If so, I have a concern with this.
That YAML file gets updated frequently (daily?) on the GitHub side, and gets a new hash every time it changes, which would mean that we would be out-of-date with it most of them time.

As it currently stands, we have the freedom and flexibility to update our YAML file when it benefits us to do so, and I would like to continue with this luxury if possible.

[alexandear]

Yes. I didn't change the previous behaviour.
.script/lint.sh validates OpenAPI every time when running on GitHub Actions:

https://github.com/google/go-github/actions/runs/23317729164/job/67821480217#step:5:21

[gmlewis]

OK, excellent! That's perfect. I just didn't want an update.

Thank you, @alexandear!
LGTM.
Merging.