mcp: add deterministic sort order and robust pagination for session listing

[eric-tramel]

Summary

Session pagination currently requires fragile cursor chaining and can make oldest/newest retrieval hard.

Request

Add stable sorting and pagination options to session listing, including:

• explicit sort (asc|desc)
• reliable pagination token/cursor behavior
• optional limit with clear upper bound

Acceptance Criteria

• Consumers can reliably fetch oldest session first/last in one pass pattern.
• Cursor behavior is deterministic and documented.
• Works consistently for very large conversation sets.