chore: update Makefile ensure_golangci-lint

[mbevc1]

Update Makefile ensure Go linting and make it more robust across platforms and environments. It might fail on non-brew ENVs.

Checklist

• Docs are autogenerated from CLI help — make sure the help text reflects your changes
• Helm chart (charts/k8s-reporter/) updated, if needed. Note: these changes live in a separate PR
• Terraform provider and related changes (terraform-provider-kosli, terraform-aws-evidence-reporter, terraform-aws-kosli-reporter) updated, if needed

[claude]

Claude finished @mbevc1's task in 51s —— View job

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Review: ensure_golangci-lint robustness

Thanks for hardening this target — it directly resolves the earlier concern about brew upgrade golangci-lint failing on a fresh macOS where the formula was never installed. The new flow is a clear improvement. ✅

What's good

• Handles the not-yet-installed case — brew ls --versions golangci-lint gates upgrade vs install, so a fresh Homebrew user no longer hits Error: golangci-lint not installed.
• Graceful non-brew fallback — Linux/CI and other non-brew environments now degrade cleanly instead of failing on a missing brew.
• Helpful failure path — when nothing is available, it prints an actionable go install command and exits non-zero (rather than silently continuing into a broken lint).
• Version consistency — GOTOOLCHAIN=go$(GO_VERSION) is derived from go.mod (go 1.26.5), and the /v2/ install path matches .golangci.yml (version: "2") and the CI golangci-lint-action@v9. All aligned. 👍

Minor / optional

• The elif command -v golangci-lint branch uses whatever binary is on PATH without checking it's a v2 build. A contributor with an old golangci-lint v1 on PATH would pass this check but then fail in lint with a config-version error. Not blocking — just a sharp edge you could smooth later (e.g. golangci-lint version sanity check) if it ever bites.

No compile/test impact — this is Makefile-only. Nice, focused change.

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• Read PR diff vs origin/main
• Review robustness of ensure_golangci-lint
• Post review
  · branch 20260803_go_lint