Trim page props and page the conversation's messages

Bot and workspace lists in page props leave out instructions, memory and
workspace context, and carry the last message as a short preview. The
header avatar's edit form loads the bot's instructions when it opens.
The conversation renders its latest 30 messages and loads older ones as
the thread scrolls to its top.

Also: the composer shows a placeholder until its editor loads, and
`bun start` no longer builds first.

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

src/utils/rail-workspaces.ts

@@ -0,0 +1,14 @@
+import type { WorkspaceWithActivity } from "@/src/types";
+
+/**
+ * Workspaces as page props carry them for the rail: without their shared
+ * context (`system_prompt`), which only the open workspace's own pages
+ * read, from their `workspace` prop.
+ */
+export default function railWorkspaces({
+    workspaces,
+}: {
+    workspaces: WorkspaceWithActivity[];
+}): WorkspaceWithActivity[] {
+    return workspaces.map(({ system_prompt, ...workspace }) => workspace);
+}