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>

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