Actor-driven change-delivery workflow routing

[attmous]

Summary

• add actor-driven workflow contract mode while preserving orchestrator-driven workflows
• route lanes from label-backed board state and keep durable workpad comments
• dispatch explicit implement/review/rework/land actor modes through the runner
• keep Python responsible for lane reliability, retries, cleanup, and durable state

Verification

• uv run ruff check packages/core/src/sprints/workflows
• uv run python -m compileall packages/core/src/sprints/workflows
• targeted routing smoke checks
• focused final review with no findings