[Feature]: Allow multi script types on same speckit project

### Problem Statement

Now speckit only generates either powershell or bash scripts. They're stored on .specify/scripts/ and declared on .specify\integrations\speckit.manifest.json. However, for a team that works with different OS's, each team member will have to generate the scripts again by running `specify init --here` on the project root, so they match the current OS.

### Proposed Solution

It makes perfect sense for speckit to generate both types of scripts and then choose the appropriate ones based on the current OS.

### Alternatives Considered

_No response_

### Component

Specify CLI (initialization, commands)

### AI Agent (if applicable)

None

### Use Cases

_No response_

### Acceptance Criteria

- [] Both powershell and bash scripts are generated on `specify init`.
- [] The speckit.manifest.json maps all scripts.
- [] The users can checkout a initialized speckit project and run specify commands and speckit workflow prompts without needed to adapt anything, independently of what OS they're using.

### Additional Context

_No response_