fix: preserve custom frontmatter fields when updating notes

Fixes #36 by modifying entity_service.update_entity() to read existing
frontmatter from files before updating them. Custom metadata fields
such as Status, Priority, and Version are now preserved when notes
are updated through the write_note MCP tool.

Added test case that verifies this behavior by creating a note with
custom frontmatter and then updating it.

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

Co-Authored-By: Claude <noreply@anthropic.com>
Signed-off-by: phernandez <paul@basicmachines.co>

src/basic_memory/services/entity_service.py

@@ -141,10 +141,23 @@ class EntityService(BaseService[EntityModel]):
         # Convert file path string to Path
         file_path = Path(entity.file_path)
 
+        # Read existing frontmatter from the file if it exists
+        existing_markdown = await self.entity_parser.parse_file(file_path)
+
+        # Create post with new content from schema
         post = await schema_to_markdown(schema)
+        
+        # Merge new metadata with existing metadata
+        existing_markdown.frontmatter.metadata.update(post.metadata)
+        
+        # Create a new post with merged metadata
+        merged_post = frontmatter.Post(
+            post.content,
+            **existing_markdown.frontmatter.metadata
+        )
 
         # write file
-        final_content = frontmatter.dumps(post, sort_keys=False)
+        final_content = frontmatter.dumps(merged_post, sort_keys=False)
         checksum = await self.file_service.write_file(file_path, final_content)
 
         # parse entity from file
@@ -309,4 +322,4 @@ class EntityService(BaseService[EntityModel]):
                 )
                 continue
 
-        return await self.repository.get_by_file_path(path)
+        return await self.repository.get_by_file_path(path)