feat: add abandoned branch detection and reporting

Automatically detects and reports abandoned branches during sync operations:

- Branches are considered abandoned if they have no commits for 6+ months
- Only reports on active repositories (main/master updated within last year)
- Shows individual reports during sync of each repository
- Displays comprehensive summary after sync-all operations
- Provides helpful cleanup commands for removing old branches

This helps maintain cleaner repositories by identifying stale branches
that may no longer be needed.

Example output:
🔍 Abandoned branches in dtail:
   Main branch last updated: 2025-04-17
   Found 1 abandoned branches (no commits for 6+ months):
   - develop (last commit: 2023-10-05, No commits for 628 days)

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

1 files changed, 16 insertions, 0 deletions

diff --git a/internal/cli/sync_handlers.go b/internal/cli/sync_handlers.go
index d14255e..dd0dcf9 100644
--- a/internal/cli/sync_handlers.go
+++ b/internal/cli/sync_handlers.go
@@ -66,6 +66,12 @@ func HandleSyncAll(cfg *config.Config, flags *Flags) int {
 	}
 	
 	fmt.Printf("\nSuccessfully synced all %d repositories!\n", successCount)
+	
+	// Print abandoned branches summary
+	if summary := syncer.GenerateAbandonedBranchSummary(); summary != "" {
+		fmt.Print(summary)
+	}
+	
 	return 0
 }
 
@@ -269,6 +275,11 @@ func syncCodebergRepos(cfg *config.Config, flags *Flags, repos []codeberg.Reposi
 	fmt.Printf("\n=== Summary ===\n")
 	fmt.Printf("Successfully synced: %d repositories\n", successCount)
 	
+	// Print abandoned branches summary
+	if summary := syncer.GenerateAbandonedBranchSummary(); summary != "" {
+		fmt.Print(summary)
+	}
+	
 	if !flags.SyncGitHubPublic {
 		return 0
 	}
@@ -304,6 +315,11 @@ func syncGitHubRepos(cfg *config.Config, flags *Flags, repos []github.Repository
 	fmt.Printf("\n=== Summary ===\n")
 	fmt.Printf("Successfully synced: %d repositories\n", successCount)
 	
+	// Print abandoned branches summary
+	if summary := syncer.GenerateAbandonedBranchSummary(); summary != "" {
+		fmt.Print(summary)
+	}
+	
 	return 0
 }