Complete planning process specification with detailed implementation design for LLM agent (#42)

* Initial plan

* Add comprehensive planning process specification in Japanese

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* Update README to reference planning process specification

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* Add planning process quick start guide

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* Final documentation updates for planning process specification

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* Add planning process completion report (specification only, no code)

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* Simplify planning specification based on review feedback

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* Update documentation with review feedback and add summary

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* Complete comprehensive planning specification with detailed implementation design

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* Update planning specification with English prompts and progress tracking

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Author: Copilot