[zzwong]

Summary

• Upgrade the shared golangci-lint binary from v2.12.2 to v2.15.0 for Go 1.27.4 compatibility. Keep the existing action implementation and lint/config gates unchanged.
• Add toolchain go1.27.4 to the minimal CI fixture while retaining its go 1.26 language floor, exercising the selected compiler through the existing cross-platform build and test-lint jobs.

Evidence and validation

• Go 1.27.4 consumer runs exposed the old binary incompatibility: slack-chat-api#219 reports that the Go version used to build golangci-lint is lower than the target; newrelic-cli#151 reports a newer-Go panic.
• v2.15.0 already passed lint on Go 1.27.4 in hubspot-cli#73 and cpm#49.
• Local YAML parsing and assertions passed for the version pin, unchanged config gate, language floor, and fixture toolchain.
• Go build/test/lint were not run locally because this environment has no Go compiler. Existing repository CI is the execution gate.
• No merge, release, deployment, or movement of the v1 tag. Consumers can test this immutable commit separately.

Fixture toolchain selection

The first shared action exports GOTOOLCHAIN=local. The test-lint job explicitly resets GOTOOLCHAIN=auto for each composite call so all test, coverage, and lint steps honor the fixture toolchain instead of later steps falling back to its language floor. This is test-only; the consumer action is unchanged from b816e09.