Add rename-prefix --repair flag and consolidate issue ID parsing

Enhances rename-prefix command with --repair flag to consolidate databases
with multiple prefixes. Creates shared issue ID utilities to eliminate code
duplication across import and rename operations.

Key changes:
- Add --repair flag to detect and consolidate multiple issue prefixes
- Create internal/utils/issue_id.go with ExtractIssuePrefix() and ExtractIssueNumber()
- Update all duplicate prefix extraction code to use shared utilities
- Add comprehensive tests for repair functionality

Generated with [Claude Code](https://claude.ai/code)
via [Happy](https://happy.engineering)

Co-Authored-By: Claude <noreply@anthropic.com>
Co-Authored-By: Happy <yesreply@happy.engineering>

internal/utils/issue_id.go

@@ -0,0 +1,26 @@
+package utils
+
+import (
+	"fmt"
+	"strings"
+)
+
+// ExtractIssuePrefix extracts the prefix from an issue ID like "bd-123" -> "bd"
+func ExtractIssuePrefix(issueID string) string {
+	parts := strings.SplitN(issueID, "-", 2)
+	if len(parts) < 2 {
+		return ""
+	}
+	return parts[0]
+}
+
+// ExtractIssueNumber extracts the number from an issue ID like "bd-123" -> 123
+func ExtractIssueNumber(issueID string) int {
+	parts := strings.SplitN(issueID, "-", 2)
+	if len(parts) < 2 {
+		return 0
+	}
+	var num int
+	fmt.Sscanf(parts[1], "%d", &num)
+	return num
+}