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>

cmd/bd/import_phases.go

@@ -9,6 +9,7 @@ import (
 	"github.com/steveyegge/beads/internal/storage"
 	"github.com/steveyegge/beads/internal/storage/sqlite"
 	"github.com/steveyegge/beads/internal/types"
+	"github.com/steveyegge/beads/internal/utils"
 )
 
 // Phase 1: Get or create SQLite store for import
@@ -58,7 +59,7 @@ func handlePrefixMismatch(ctx context.Context, sqliteStore *sqlite.SQLiteStorage
 
 	// Analyze prefixes in imported issues
 	for _, issue := range issues {
-		prefix := extractPrefix(issue.ID)
+		prefix := utils.ExtractIssuePrefix(issue.ID)
 		if prefix != configuredPrefix {
 			result.PrefixMismatch = true
 			result.MismatchPrefixes[prefix]++
@@ -350,14 +351,6 @@ func importComments(ctx context.Context, sqliteStore *sqlite.SQLiteStorage, issu
 
 // Helper functions
 
-func extractPrefix(issueID string) string {
-	parts := strings.SplitN(issueID, "-", 2)
-	if len(parts) < 2 {
-		return "" // No prefix found
-	}
-	return parts[0]
-}
-
 func getPrefixList(prefixes map[string]int) []string {
 	var result []string
 	keys := make([]string, 0, len(prefixes))