Fix: Prevent reinitialization of existing ProllyTree stores

[zhangfengcdt]

Summary

• Fixed init function in versioned_store.rs to detect and load existing stores instead of reinitializing them
• Prevents data loss when init is called on directories with existing ProllyTree data
• Adds proper test coverage for this scenario

Problem

The init function was always creating a new store with default configuration and an "Initial commit", even when prolly config files already existed in the directory. This could lead to:

• Data loss as existing trees would be overwritten
• Duplicate "Initial commit" entries in git history
• Confusion when existing stores appeared to be reset

Solution

Added a check in the init function to detect existing prolly config files (prolly_config_tree_config and prolly_hash_mappings). When either file exists, the function now calls open() to load the existing configuration instead of creating a new one.

Test Plan

• Added unit test test_init_with_existing_store that verifies data persistence across multiple init calls
• Manually tested with git-prolly CLI to confirm existing data is preserved
• All existing tests pass
• cargo fmt and clippy checks pass

Example

# First initialization
git-prolly init
git-prolly set key1 value1
git-prolly commit -m "Add key1"

# Second init (previously would reset, now loads existing)
git-prolly init
git-prolly get key1  # Returns "value1" - data preserved!