go/lint: move gofmt into golangci-lint and add ONLY_GOLANGCI flag

[rayjohnson]

Summary

• Removes the standalone gofmt check and adds gofmt (with simplify: true) as a linter within golangci-lint
• Adds ONLY_GOLANGCI=yes flag to skip all other checks (dependency validation, retracted modules, build, tests) and run only golangci-lint

Test plan

• Run ONLY_GOLANGCI=yes bash lint-project.sh and verify only golangci-lint runs
• Run bash lint-project.sh normally and verify gofmt formatting is still checked via golangci-lint

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request refactors the linting script to replace the manual gofmt check with the golangci-lint implementation and introduces an ONLY_GOLANGCI flag to skip auxiliary checks like gitleaks, govulncheck, and dependency verification. A review comment suggests also clearing the EXPERIMENTAL variable when ONLY_GOLANGCI is enabled to ensure experimental linters are correctly bypassed.

[adamdecaf]

Error: can't load config: gofmt is a formatter
The command is terminated due to an error: can't load config: gofmt is a formatter

[rayjohnson]

Tested in a couple repos - all seems good.