feat: runtime-truth task decomposition and lane-aware sprint artifacts

[graysurf]

Summary

• Populate runtime-truth validation during start-plan issue-body generation before live writes.
• Generate sprint task-spec and subagent prompts from issue-row runtime metadata after sync.
• Add lane-aware prompt deduplication so shared runtime lanes emit one prompt with task list and execution mode.
• Add/rename Sprint 3 regression tests for S3T1 and S3T2 command-level validation.

Scope

• Included:
  • crates/plan-issue-cli/src/execute.rs
  • crates/plan-issue-cli/tests/sprint3_delivery.rs
  • crates/plan-issue-cli/tests/task_spec_flow.rs
• Excluded:
  • Sprint 3 Task S3T3 drift-enforcement behavior changes.

Testing

• cargo test -p nils-plan-issue-cli --test sprint3_delivery start_plan_dry_run_writes_runtime_truth_task_decomposition_metadata -- --exact (pass)
• cargo test -p nils-plan-issue-cli write_subagent_prompts_groups_tasks_by_runtime_lane -- --exact (pass)
• cargo test -p nils-plan-issue-cli --test task_spec_flow (pass)

Issue

Closes #250 (tasks S3T1, S3T2)