Gateway: keep whole T3 history pages, and show a command's first line as its activity

docs/api.md

@@ -94,7 +94,7 @@ when the gateway cannot find its parent, or the parent already shows a request.
 for a moment after a thread changes.
 
 `activity` is the newest step the agent has in progress. `kind` is `command`, `edit`, `search`,
-`web`, `subagent`, `thinking` or `tool`, and `text` is the command line, the file name (without its
+`web`, `subagent`, `thinking` or `tool`, and `text` is the command's first line, the file name (without its
 directory), the search, the subagent's title or the tool's name, trimmed to about 80 characters.
 Monitors are left out, since they run for hours. It is null when no step is in progress. The gateway
 rereads a running thread at most every 5 seconds, so it can lag that much.
@@ -136,7 +136,8 @@ GET /v1/threads/:hostId/:threadId/history?cursor=<earlierCursor>
 200 { "messages": [Message], "earlierCursor": "..." | null }
 ```
 
-The 20 messages before the ones the watch has, oldest first. Pass the returned `earlierCursor` for
+The messages before the ones the watch has, oldest first: 20 at a time at first, then one T3 history
+page at a time, which can hold more. Pass the returned `earlierCursor` for
 the page before that. It is null at the start of the thread. Cursors are opaque.
 
 ```