Improve plan skill with superpowers patterns

[HumanBean17]

Summary

• Add No placeholders section: explicit ban on TBD, vague directives, undefined references, with a DEFERRED: escape hatch for genuinely deferred decisions
• Add Self-review step: four checks (spec coverage, placeholder scan, cross-PR consistency, dependency order) to run before finalizing a plan
• Add Execution handoff section: three modes (subagent-driven, inline, manual) with superpowers skill references
• Tighten final checklist to include named tests, no-placeholder verification, and consistency checks

Patterns adopted from the superpowers writing-plans skill, adapted to the repo's PR-level planning abstraction.

Test plan

• Verify SKILL.md renders correctly
• Use the skill on a real plan and confirm the new sections produce higher-quality output

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