[Feature]: stage wise constitution

### Problem Statement

Currently we have one constitution file and if I include a lot of instructions in that file, there is a tendency for speckit to ignore some of them. And all the instructions may not be relevant for all stages of a project. For e.g. during requirements, only instructions related to defining the requirements are relevant. During the implement, only instructions related to coding are relevant.

### Proposed Solution

we can have multiple constitution files e.g. constitution-specify.md, constitution-plan.md, constitution-tasks.md, constitution-implement.md etc. and each file can have instructions that are specific to the requirements, design, task breakdown, implementation stages.
This will require the commands of those specific stages to be modified to explicitly read those constitution files for a consistent experience.

### Alternatives Considered

_No response_

### Component

Specify CLI (initialization, commands)

### AI Agent (if applicable)

None

### Use Cases

_No response_

### Acceptance Criteria

_No response_

### Additional Context

_No response_

まず、Specify CLI の初期化フローと、issue で指定されているステージ固有のコマンド（specify、plan、tasks、implement）を追跡します。次に、現在の Constitution ファイルがどこで読み込まれているかを特定し、各ステージがどのように指示を選択すべきかを決定します。完了とは、提案されたステージ固有の Constitution の動作が、初期化および一覧にあるすべてのコマンド全体で一貫して定義されていることを意味します。

索引モデルが issue の本文から書いたものです。