Use golangci-lint fmt to format code (#823)

Use `golangci-lint fmt` to format code, replacing the previous gofumpt-based formatter. https://github.com/daixiang0/gci is used to order the imports.

Also drops the `gitea-vet` dependency since `gci` now handles import ordering.

Mirrors https://github.com/go-gitea/gitea/pull/37194.

---
This PR was written with the help of Claude Opus 4.7

Reviewed-on: https://gitea.com/gitea/act_runner/pulls/823
Reviewed-by: Nicolas <[email protected]>
Co-authored-by: silverwind <[email protected]>
Co-committed-by: silverwind <[email protected]>

.gitignore

@@ -2,7 +2,6 @@
 .env
 .runner
 coverage.txt
-/gitea-vet
 /config.yaml
 
 # Jetbrains