build: bump to go1.25.8

[fin09pcap]

Notes for Reviewer

• Bump Go toolchain to 1.25.8 (terraformer/.tool-versions, go.mod)
• Refresh .golangci.yml for current linter versions
• Fix HCL format error reporting in terraformer/pkg/terraformer/hcl/format.go — hclwrite.Format silently ignores syntax errors, so the previous if err != nil branch was dead code.
  Now parse with hclwrite.ParseConfig first and return the hcl.Diagnostics when present
• Modernize release-harp-terraformer.yml — upgrade actions/checkout@v4, setup-go@v5 (reads go-version-file), setup-buildx-action@v3, build-push-action@v6, pin cosign-installer@v3.8.2 and upload-release-assets by SHA; fix release-tag prefix to terraformer/

What issues does this PR fix?

Rel:

• Routine Go 1.25.8 toolchain + CI maintenance for the terraformer plugin
• Surfaces HCL syntax diagnostics that were previously swallowed during harp-terraformer output formatting