feat: bound MCP collection pagination

[dajiaohuang]

Summary

• add scope-bound opaque cursors, bounded filtering, page, and next_cursor across growing MCP list/search/query facades while preserving result and text compatibility
• push event and revision continuation offsets into SagaSmith Core with limit + 1 look-ahead, including regression coverage beyond record 100, and pin locked CI to Core 612bfe7e5290eb5b23f2811baa83b8a28293b36e
• replace nominal read-only evaluations with ten independent, actually solved multi-page and multi-hop evaluations over six campaigns and 30 actors
• document the collection contract, Host-safe cursor rules, compatibility payload fields, and evaluation acceptance threshold

Validation

• ruff check packages/domain packages/mcp
• python -m pytest packages/domain/tests -q (passed)
• python -m pytest packages/mcp/tests -q (passed against the pinned Core pagination change; one existing skip)
• after rebasing current origin/main, focused actor-audience, pagination, evaluation, MCP 2026, and tool-contract suites passed
• no paid model, production data, secret, or external service used

Depends on merged Core PR SagaSmithAI/Sagasmith-core#18.