Bump golangci-lint to 2.15.0

With Go 1.27 installed, `just lint` fails with typecheck errors like
"export data version 5 is greater than maximum supported version 4".
The shim builds golangci-lint with the local Go, but v2.12.2 can't read
the export data of Go 1.27's standard library. Support for Go 1.27 came
in v2.13.0.

v2.15.0 still builds with Go 1.26, so linting keeps working there too.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -195,7 +195,7 @@ jobs:
         uses: golangci/golangci-lint-action@ba0d7d2ec06a0ea1cb5fa41b2e4a3ab91d21278a # v9
         with:
           # If you change this, make sure to also update scripts/golangci-lint-shim.sh
-          version: v2.12.2
+          version: v2.15.0
   upload-coverage:
     # List all jobs that produce coverage files
     needs: [unit-tests, integration-tests]

scripts/golangci-lint-shim.sh

@@ -3,6 +3,6 @@
 set -e
 
 # Must be kept in sync with the version in .github/workflows/ci.yml
-version="v2.12.2"
+version="v2.15.0"
 
 go run "github.com/golangci/golangci-lint/v2/cmd/golangci-lint@$version" "$@"