test: enforce start-sprint runtime-truth drift gate

[graysurf]

Summary

• Add an end-to-end dry-run test that verifies start-sprint derives artifacts from issue-table rows.
• Assert drift between issue-table rows and plan-derived rows returns task-sync-drift-detected.
• Align runtime-lane prompt expectation in execute unit tests for multi-group auto mode.

Testing

• cargo test -p nils-plan-issue-cli --test sprint3_delivery start_sprint_uses_issue_table_runtime_truth_and_rejects_drift -- --exact
• cargo test -p nils-plan-issue-cli write_subagent_prompts_groups_tasks_by_runtime_lane -- --exact

Issue

Refs #250 (task S3T3)