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>

1 files changed, 19 insertions, 0 deletions

diff --git a/README.md b/README.md
index 640ccab..2cb27e9 100644
--- a/README.md
+++ b/README.md
@@ -111,6 +111,25 @@ Create a `gitsyncer.json` file:
    - Merges changes from remotes that have the branch
    - Pushes to all remotes (creating branches if needed)
 
+## Branch Exclusion
+
+You can exclude branches from synchronization using regex patterns in your configuration:
+
+```json
+{
+  "organizations": [...],
+  "repositories": [...],
+  "exclude_branches": [
+    "^codex/",    // Exclude branches starting with "codex/"
+    "^temp-",     // Exclude branches starting with "temp-"
+    "-wip$",      // Exclude branches ending with "-wip"
+    "experimental" // Exclude branches containing "experimental"
+  ]
+}
+```
+
+Excluded branches will be reported during sync but not synchronized.
+
 ## Example Workflows
 
 ### Sync specific repositories