Adds .github/workflows/ci.yml so PRs touching Go code are validated
independently of the release workflow. Previously the test job was
co-located inside release.yml gated on `startsWith(github.ref, 'refs/tags/v')`,
meaning no automated test ran on PRs until a tag was pushed.
The new workflow mirrors the release.yml test job (go mod tidy, go vet, go test
-race) and triggers on every push to main and every pull request that touches
Go code, using Go 1.25 to match release.yml.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>