COL-14 remove retired go report badge from readme

[cvsouth]

goreportcard.com is confirmed sunset: its front page now reads "After more than a decade, Go Report Card has been sunset."

I removed the badge from README.md and also the ## Go Report Card section from CLAUDE.md, which instructed maintaining an A+ grade on the same retired service. That is slightly wider than the ticket's wording, but everything that section enforced is already in the CI section above it (gofmt, gocyclo, ineffassign, misspell, LICENSE), and go_vet is covered by golangci-lint's default govet linter, so nothing enforceable was lost.

All CI checks pass: gofmt, go vet, golangci-lint (0 issues), gocyclo, ineffassign, misspell, govulncheck (0 vulnerabilities called), and the full test suite.

One environment problem, unrelated to this change and worth fixing separately: go.mod now requires Go 1.26.7 (commit 40a6621) but the system Go here is 1.26.5, and go cannot download the toolchain because IPv6 egress gets 403 Forbidden from proxy.golang.org (IPv4 returns 200). Nothing in this checkout builds with the default toolchain as a result. I fetched 1.26.7 over IPv4 into /tmp to run the checks and deleted it afterwards; no system state was changed.

Go Report Card has been sunset, so the badge no longer reports anything. Removed it from the README, and also removed the CLAUDE.md section instructing that the project hold an A+ grade there, since that instruction can no longer be acted on. The checks it named are already covered by the CI section above it: gofmt, gocyclo, ineffassign and misspell are listed there directly, and go vet runs as golangci-lint's default govet linter. No code changed.