Add a cursor query parameter to GET /jobs and return a next_cursor
field in the response envelope. Existing clients using only limit
continue to work without modification. The cursor is an opaque token
encoding created_at and job_id for stable keyset pagination.
Closesleeworks-agents/SPARC#25
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>