Extend the `documents.update` API endpoint with a `find_replace` editMode to allow surgical edits to existing content without replacing the entire document body. This is needed for API integrations and LLM-powered workflows to avoid race conditions with live editors.
### Problem The `documents.update` endpoint currently supports three `editMode` values: `replace`, `append`, and `prepend`. These work well for full-document updates or adding new content, but there's no way to make **surgical edits to existing content** without replacing the entire document body. This creates a real problem for API integrations and LLM-powered workflows: 1. **Race conditions with live editors.** If a user is editing in the browser while an API client updates the document, the API client must fetch → modify → push the full text, overwriting any concurrent browser changes. 2. **Unnecessary revision noise.** A one-line fix creates a full-document revision, making revision history harder to audit. 3. **No atomic partial edits.** The workaround (fetch full text, string-replace client-side, push full text) is fragile and not atomic. ### Proposed Solution Add a new `editMode: "find_replace"` option to `documents.update`: ```json POST /api/documents.update { "id": "do