refactor: harden implementation-plan contract for lower-tier execution

[IgorWnek]

This PR hardens the implementation-plan workflow so weaker implementor models can execute plans more mechanically and safely, without having to reverse-engineer the full document. The changes are additive and structural only across five existing artifacts: the plan template, the implementation-plan skill, the orchestration skill, the review-plan prompt, and the published documentation.

What will change

• Add per-task Files: anchors so each task explicitly names the files it will create, modify, or reuse.
• Require at least one runnable command in every task’s Definition of Done so reviewers can verify completion concretely.
• Add an optional Stop Rule: at task level so implementors know when to stop and report instead of improvising.
• Add per-phase Verification: commands with fast checks that should run after each phase.
• Strengthen the Wildly Important Goal guidance with an explicit success measure and non-goals to reduce scope creep.
• Refine the no-real-code rule so it still bans implementation bodies and function logic, while explicitly allowing boundary artifacts such as type definitions, interfaces, DTOs, signatures, and API shapes.
• Add a hard open-questions dispatch gate so plans cannot move forward while any ## Open Questions row still has status ❓ Open.