Fix: Prevent reinitialization of existing ProllyTree stores (#100)

The init function in versioned_store.rs was creating a new store even when one already existed, potentially causing data loss. This fix adds a check for existing prolly config files before initialization.

Changes:
- Added check for prolly_config_tree_config and prolly_hash_mappings files in init()
- If either file exists, calls open() to load the existing store instead of creating a new one
- Added comprehensive test to verify the behavior

This ensures that repeatedly calling init on the same directory preserves existing data and doesn't create duplicate Initial commit entries.

Fixes the issue where init would overwrite an existing store configuration.

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

Co-authored-by: Claude <noreply@anthropic.com>

Author: zhangfengcdt

src/git/versioned_store.rs

@@ -1181,6 +1181,15 @@ impl<const N: usize> VersionedKvStore<N, GitNodeStorage<N>> {
             )
         })?;
 
+        // Check if the store is already initialized by looking for config files
+        let config_path = path.join("prolly_config_tree_config");
+        let mappings_path = path.join("prolly_hash_mappings");
+
+        if config_path.exists() || mappings_path.exists() {
+            // Store already exists, use open instead to load existing configuration
+            return Self::open(path);
+        }
+
         // Open the existing git repository
         let git_repo = gix::open(&git_root).map_err(|e| GitKvError::GitOpenError(Box::new(e)))?;
 
@@ -2673,6 +2682,46 @@ mod tests {
         }
     }
 
+    #[test]
+    fn test_init_with_existing_store() {
+        let temp_dir = TempDir::new().unwrap();
+
+        // Initialize git repository
+        gix::init(temp_dir.path()).unwrap();
+
+        // Create subdirectory for dataset
+        let dataset_dir = temp_dir.path().join("dataset");
+        std::fs::create_dir_all(&dataset_dir).unwrap();
+
+        // First init - creates new store
+        let mut store = GitVersionedKvStore::<32>::init(&dataset_dir).unwrap();
+
+        // Add some data and commit
+        store
+            .insert(b"test_key".to_vec(), b"test_value".to_vec())
+            .unwrap();
+        store.commit("Test data").unwrap();
+
+        // Drop the store to ensure we're testing a fresh init
+        drop(store);
+
+        // Second init - should load existing store, not create new one
+        let store2 = GitVersionedKvStore::<32>::init(&dataset_dir).unwrap();
+
+        // Verify data still exists (wasn't overwritten)
+        assert_eq!(
+            store2.get(b"test_key"),
+            Some(b"test_value".to_vec()),
+            "Data should still exist after second init"
+        );
+
+        // Verify config files exist
+        let config_path = dataset_dir.join("prolly_config_tree_config");
+        let mapping_path = dataset_dir.join("prolly_hash_mappings");
+        assert!(config_path.exists(), "Config file should exist");
+        assert!(mapping_path.exists(), "Mapping file should exist");
+    }
+
     #[test]
     fn test_diff_with_no_changes() {
         let temp_dir = TempDir::new().unwrap();