mcp: add get_session_events API for full session timeline

[eric-tramel]

Summary

There is no MCP endpoint to fetch a full ordered event timeline for a session, requiring low-level datastore access for basic transcript reconstruction.

Request

Add a dedicated MCP endpoint/tool:

• get_session_events
• Inputs: session_id, optional limit, cursor, direction
• Output: ordered event rows with event identifiers, timestamps, event kinds/actors, and message/payload references.

Acceptance Criteria

• Returns a stable, deterministic stream of events for a session.
• Supports forward and reverse pagination.
• Optionally supports lightweight filtering by event kind when practical.
• Enables complete conversation inspection without direct DB queries.