Extract normalizeLabels to internal/util/strings.go

- Created internal/util/strings.go with NormalizeLabels function
- Added comprehensive tests in internal/util/strings_test.go
- Updated internal/rpc/server_issues_epics.go to use util.NormalizeLabels
- Updated cmd/bd/list.go and cmd/bd/ready.go to use util.NormalizeLabels
- Updated cmd/bd/list_test.go to use util.NormalizeLabels
- Removed duplicate implementations
- All tests pass

Fixes bd-fb95094c.6

Amp-Thread-ID: https://ampcode.com/threads/T-edb3c286-cd60-4231-94cd-edaf75d84a3d
Co-authored-by: Amp <amp@ampcode.com>

internal/util/strings.go

@@ -0,0 +1,22 @@
+package util
+
+import "strings"
+
+// NormalizeLabels trims whitespace, removes empty strings, and deduplicates labels
+// while preserving order.
+func NormalizeLabels(ss []string) []string {
+	seen := make(map[string]struct{})
+	out := make([]string, 0, len(ss))
+	for _, s := range ss {
+		s = strings.TrimSpace(s)
+		if s == "" {
+			continue
+		}
+		if _, ok := seen[s]; ok {
+			continue
+		}
+		seen[s] = struct{}{}
+		out = append(out, s)
+	}
+	return out
+}