New feature - Workflow UPDATE_RECORD allow different update modes (replace / append / only-if-null)

Summary UPDATE_RECORD always replaces a field's value. There's no per-field write mode, so common needs — append to a multi-select, or fill only when the field is empty — each require a workaround: a FIND_RECORDS to read the current value, a CODE step to merge, then the UPDATE_RECORD. Motivating example To keep a Company's companySources (multi-select) in sync as people are attributed to it, we have to: FIND the company → CODE step that unions existing + incoming values and dedupes → UPDATE. The exact same three-step pattern is repeated for warehouse and biTool. All of it would collapse into a single "append (dedupe)" write. Proposal Add a per-field write mode on UPDATE_RECORD: • replace — current behavior (default) • append — for array / multi-select fields, with an optional dedupe • only-if-null / fill-if-empty — don't overwrite an existing value (protects human- or other-source-entered data) Benefit Removes the read → merge → …