Replace the composer textarea with a Tiptap markdown editor

The composer now writes markdown with bold, italic, strike, code,
links, quotes and lists, from shortcuts, markdown typing or a bubble
over the selection. @ mentions a bot and # a workspace, shown as chips
and saved as [bot:ID] / [workspace:ID]. Text selected in a message can
be quoted into the composer. User messages render as markdown, and
replies tell the bot who and what the user mentioned.

Adds an ESM stand-in for use-sync-external-store, since bunext keeps
React external in page bundles and the CommonJS original requires it.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

src/utils/message-preview.ts

@@ -1,13 +1,15 @@
 /**
  * A bot message as one line of plain text, for the rail's preview: no
  * markdown syntax, links as their text, computer refs (`computerRef`) as
- * "a computer", bot refs (`botRef`) as "a bot", whitespace collapsed.
+ * "a computer", bot refs (`botRef`) as "a bot", workspace refs
+ * (`workspaceRef`) as "a workspace", whitespace collapsed.
  */
 export default function messagePreview({ text }: { text: string }) {
     return text
         .replace(/```[^\n]*\n?/g, "")
         .replace(/\[computer:\d+\](?!\()/g, "a computer")
         .replace(/\[bot:\d+\](?!\()/g, "a bot")
+        .replace(/\[workspace:\d+\](?!\()/g, "a workspace")
         .replace(/!?\[([^\]]*)\]\([^)]*\)/g, "$1")
         .replace(/^\s{0,3}(#{1,6}\s+|>\s?|[-*+]\s+|\d+\.\s+)/gm, "")
         .replace(/(\*\*|__|~~|`)/g, "")