Add history pagination to the agent command center - #49106

## Why

The command center seeded only ten recent sessions, with no way to browse older history from the task list.

## What changed

- Add a keyboard-selectable “Show more” row that discovers up to ten more tasks, with loading and retry states that remain accessible during search.
- Retain per-source cursors and buffered rows across requests, merging interactive and non-interactive history by recency without discarding existing tasks on failure.
- Refill vacancies after tasks are archived or deleted, and prevent stale refreshes from restoring removed tasks.
- Limit routine detail refreshes to recent and loaded tasks, plus explicitly targeted threads, as retained history grows.

## Testing

Add regression coverage for pagination across mixed sources, cursor retries without row loss, automatic refill, queued pagination, and “Show more” navigation, loading, search, and retry rendering.

GitOrigin-RevId: 0bfcc970c9adf0c8d509b275a92ecf01c29595ad