[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_

Start by tracing the Specify CLI initialization flow and the stage-specific commands named in the issue: specify, plan, tasks, and implement. Then identify where the current constitution file is loaded and determine how each stage should select its instructions. Done means the proposed stage-specific constitution behavior is defined consistently across initialization and all listed commands.

由索引模型根据 Issue 内容生成。