EXAMPLES:

  # Replace a document with a fixed string
  iwe update -k notes/draft -c "# Draft\n\nNew content."

  # Pipe content from another command
  cat new-content.md | iwe update -k notes/draft -c -

  # Preview without writing
  iwe update -k notes/draft -c "..." --dry-run

NOTES:

  - The target document must already exist; update does not create new docs.
  - Use `iwe new` to create documents from a template.
  - Use `iwe attach` to append a block reference instead of overwriting.
