Page list tools with one limit and cursor contract #304

See #302. One contract for every list tool, described once in the system guide.

Contract

• Request: limit (small default, documented maximum) and cursor from the previous response.
• Response: entries plus total, hasMore and nextCursor.
• Deterministic order with a stable tie-break (as #116 already does), so pages have no gaps or duplicates.
• A cursor is opaque, bound to the caller, the query and its sort; a cursor used with different arguments is refused rather than guessed. Define what happens when items change between pages and test it.

Work

• Apply to task_list, item_search (per item kind or one merged sequence, decide and document) and the proposal_get queue; align item_history and relation_list, which already take limit.
• Replace the fixed caps (300 root tasks in TaskService.List, 30/20/20 in item_search) with the contract.
• #219 builds the complete memory inventory on this contract; bag children (#295) use it too.

Acceptance

• Exhausting a fixture larger than the maximum limit returns every visible entry exactly once, including equal timestamps and subtasks that follow their parent across a page boundary.
• total and cursors reveal nothing the caller may not read.
• Tool list stays within budget or the budget is raised deliberately with the reason recorded in McpDiscoveryTests.