Slash commands and plugins can now append or insert into the input box, not just replace it

What

The internal system plugins and hooks use to put text into your input box (the "draft") has been extended.

• The fillDraft API, used by plugins and hooks to populate the input box, now takes a mode: replace (the old default, replaces everything), append (adds text to the end of what's already there), or insert (inserts text at your cursor position).
• A companion readDraft API (also exposed as the hook method $.prompt.read) lets a plugin read the current input box text and cursor position before deciding what to do.
• Slash-command-triggered plugin actions of type prompt.fill (defaulting to replace mode) and prompt.suggest now get the same default plugin-origin metadata that command.run actions already had.
• A new settings layer called memory was added alongside the existing managed/user/project/local layers, and the replace/append/insert modes are validated against an explicit list, with an error shown for invalid modes.

Why

This gives plugins finer control over how they interact with whatever you're already typing, instead of always overwriting it. A plugin can now add a suggestion after your text or insert something right where your cursor is, and it can check what's already in the box first.