Migrate lint job to golangci-lint v2

[dolph]

Why

The lint check fails on both open pull requests (#105, #109), and would fail on any new one:

• Update module golang.org/x/tools to v0.52.0 #105 (and main): golangci-lint-action@v9 refuses to install the pinned v1.64.8 — "golangci-lint v1 is not supported by golangci-lint-action >= v7".
• Update dependency golangci/golangci-lint to v2.15.0 #109 bumps to v2, which then reports 33 errcheck findings. golangci-lint v1 excluded unchecked Close, os.Remove(All), fmt.Fprint* and similar by default; v2 dropped those defaults.

What

• Pin golangci-lint to v2.14.0 in .github/workflows/golangci-lint.yml.
• Add .golangci.yml (v2 format) enabling the std-error-handling exclusion preset, which restores the v1 default behavior. No code changes.

I chose the preset over sprinkling _ = across 33 call sites (26 of them os.Remove cleanup in tests): it keeps the lint bar exactly where it was before, with no diff noise. If you'd rather tighten errcheck later, removing the preset is a one-line change that deserves its own PR.

Verification

Run locally with golangci-lint v2.14.0: config verify passes, golangci-lint run ./... reports 0 issues, and gofmt, go vet, go build, and go test -race are clean.

After merging

• Update dependency golangci/golangci-lint to v2.15.0 #109 becomes redundant (same version bump); Renovate should close it once main matches, or close it by hand.
• Update module golang.org/x/tools to v0.52.0 #105 only needs a rebase onto main to pick up the fix.

🤖 Generated with Claude Code

https://claude.ai/code/session_01AZrGfLRy5eYPy5v8JjKwrN

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