cmd/sgai: implement parallel agent batch workflow state

[ucirello]

Summary

• follow the GOAL truth table by prioritizing coordinator work when it has unread messages and otherwise launching unread top-level recipients as a parallel batch, while leaving multi-model sibling scheduling sequential
• keep per-agent batch state isolated across completion status, task/TODO visibility, and queued prompt-token human input so the workflow only advances after the active batch finishes
• surface the broader batch/session state through the API and session UI with grouped active-agent TODO sections, cost breakdowns, project-management content, and the prompt/snippet guidance needed for the new workflow behavior

Test Plan

• make absorb-sgai