* feat: add update_mode='append' for retain to concatenate content to existing documents
When retaining with update_mode='append' and a document_id that already exists,
the new content is appended to the existing document text and the full document
is reprocessed. Delta retain automatically skips unchanged chunks, so only the
new content triggers LLM extraction.
- Add update_mode field to MemoryItem (API), RetainContentDict (internal), MCP tools
- Validate that update_mode='append' requires a document_id
- Fetch existing document content and prepend before processing in orchestrator
- Update Python, TypeScript, Go generated clients and top-level client wrappers
- Add tests for append, multiple appends, no-existing-doc, validation, and default replace
* fix: add update_mode field to Rust CLI and client MemoryItem initializers
* chore: regenerate docs skill references for update_mode
3c633e5e16