feat: add cursor-paged listTasksPage and TaskPage type

[pikann]

Summary

Adds cursor-paged task listing to PluginApiClient, matching the host's listTasksPage() from Paca-AI/paca#598. listTasks() previously stopped silently at the first 200 tasks; there was no way to read the rest.

Changes

• listTasksPage(filters?) (new) returns one page, { items, page_size, next_cursor, total_count? }. Pass next_cursor as filters.cursor to get the next page. page_size defaults to 200.
• listTasks(filters?) now calls listTasksPage() and returns its items. It still returns a plain array, so existing callers keep working, but it is the first page only. The docs and JSDoc now say so.
• Types: new exported TaskPage; TaskFilters gains cursor.
• TaskFilters.page is marked @deprecated. The tasks endpoint pages by cursor and ignores it, so the client no longer sends it.
• README updated for the new method and the first-page behavior of listTasks().

Usage

let cursor: string | undefined;
const all = [];
do {
  const page = await api.listTasksPage({ cursor, page_size: 100 });
  all.push(...page.items);
  cursor = page.next_cursor ?? undefined;
} while (cursor);

Compatibility

Backwards compatible for callers of listTasks(): same return type. Callers that passed page get the first page instead of page N, which the API already did. Code that needs more than 200 tasks should move to listTasksPage().

Notes

• Pairs with the host change in feat: enhance paginations paca#598 (apps/web plugin client gains the same listTasksPage()).
• Needs a release before plugins can use listTasksPage().