feat: add branch exclusion feature with regex patterns

Users can now exclude branches from synchronization using regex patterns
in the configuration file. This is useful for:
- Excluding temporary or experimental branches
- Skipping vendor or third-party branches
- Ignoring deployment-specific branches

Configuration example:
```json
{
  "exclude_branches": [
    "^codex/",    // Exclude branches starting with "codex/"
    "^temp-",     // Exclude branches starting with "temp-"
    "-wip$"       // Exclude branches ending with "-wip"
  ]
}
```

Features:
- Regex pattern matching for flexible exclusion rules
- Clear reporting of excluded branches during sync
- Excluded branches are filtered from sync but still analyzed for abandonment
- Invalid regex patterns are reported but don't stop sync

The feature helps maintain cleaner synchronization by allowing users to
ignore branches that shouldn't be synchronized across all repositories.

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

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

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