ax(mining): rename cb to circuitBreaker in FetchLatestGitHubVersion

AX Principle 1 — predictable names over short names. The local variable
`cb` requires mental mapping; `circuitBreaker` names the value directly.

Co-Authored-By: Charon <charon@lethean.io>

pkg/mining/version.go

@@ -36,16 +36,16 @@ type GitHubRelease struct {
 
 // tag, err := FetchLatestGitHubVersion("xmrig", "xmrig") // "v6.21.0"
 func FetchLatestGitHubVersion(owner, repo string) (string, error) {
-	cb := getGitHubCircuitBreaker()
+	circuitBreaker := getGitHubCircuitBreaker()
 
-	result, err := cb.Execute(func() (interface{}, error) {
+	result, err := circuitBreaker.Execute(func() (interface{}, error) {
 		return fetchGitHubVersionDirect(owner, repo)
 	})
 
 	if err != nil {
 		// If circuit is open, try to return cached value with warning
 		if err == ErrCircuitOpen {
-			if cached, ok := cb.GetCached(); ok {
+			if cached, ok := circuitBreaker.GetCached(); ok {
 				if tagName, ok := cached.(string); ok {
 					return tagName, nil
 				}