Harden runtime action contracts

[pmbstyle]

Summary

• Enforce runtime action contracts before final replies so pending plan steps must be backed by executor state or an explicit terminal status.
• Replace phrase-based scheduled-control and worker-follow-up handling with explicit structured contracts.
• Require explicit worker capabilities and required tool calls instead of inferring write or vision needs from task wording.

Validation

• uv run pytest -> 903 passed, 1 skipped
• Targeted router, scheduler, worker routing, child-worker, and parallel-worker tests passed during development
• ruff check and git diff --check passed during development