Function update_memory_bank_entry 

pub async fn update_memory_bank_entry(
    workspace_dir: &Path,
    file_path: &Path,
    entry: &MemoryBankEntry,
) -> Result<(), MemoryBankError>

Update a single memory bank entry in-place.

This will validate that file_path is a markdown file located under the workspace’s .gestura/memory/ directory, then rewrite the file contents.