[Extension]: Add spec-kit-switch

### Extension ID
switch

### Extension Name
Spec Switcher

### Version
1.0.0

### Description
Navigate between specs — list available specs and switch active feature with git checkout

### Author
Wagner Sousa

### Repository URL
https://github.com/wagner-sousa/spec-kit-switch

### Download URL
https://github.com/wagner-sousa/spec-kit-switch/archive/refs/tags/v1.0.0.zip

### License
MIT

### Homepage
https://github.com/wagner-sousa/spec-kit-switch

### Documentation URL
https://github.com/wagner-sousa/spec-kit-switch#readme

### Changelog URL
https://github.com/wagner-sousa/spec-kit-switch/blob/main/CHANGELOG.md

### Required Spec Kit Version
>=0.4.0

### Number of Commands
2

### Number of Hooks
0

### Tags
navigation, workflow, specs, sdd, git

### Key Features
- List all available specs with branch and active status in a table
- Switch between specs via git checkout (avoids feature.json conflicts)
- Auto-unlock previous spec lifecycle on switch via cross-plugin interface
- --local-only flag for branchless specs

### Testing Checklist
- [x] Extension installs successfully via download URL
- [x] All commands execute without errors
- [x] Documentation is complete and accurate
- [x] No security vulnerabilities identified
- [x] Tested on at least one real project

### Submission Requirements
- [x] Valid `extension.yml` manifest included
- [x] README.md with installation and usage instructions
- [x] LICENSE file included
- [x] GitHub release created with version tag
- [x] All command files exist and are properly formatted
- [x] Extension ID follows naming conventions (lowercase-with-hyphens)

### Testing Details
Tested on Linux with Spec Kit v0.13.3 in the spec-kit-docker template project. Both commands verified: switch.list shows all specs with branch/status table, switch.set performs git checkout with auto-unlock of previous spec. Cross-plugin interface tested with lifecycle extension for seamless spec transitions.

### Example Usage
```bash
specify extension add switch --from https://github.com/wagner-sousa/spec-kit-switch/archive/refs/tags/v1.0.0.zip
/speckit.switch.list
/speckit.switch.set 013
```

### Proposed Catalog Entry
```json
{
"switch": {
"name": "Spec Switcher",
"id": "switch",
"description": "Navigate between specs — list available specs and switch active feature with git checkout",
"author": "Wagner Sousa",
"version": "1.0.0",
"download_url": "https://github.com/wagner-sousa/spec-kit-switch/archive/refs/tags/v1.0.0.zip",
"repository": "https://github.com/wagner-sousa/spec-kit-switch",
"homepage": "https://github.com/wagner-sousa/spec-kit-switch",
"license": "MIT",
"requires": {
"speckit_version": ">=0.4.0"
},
"provides": {
"commands": 2
},
"tags": ["navigation", "workflow", "specs", "sdd", "git"],
"verified": false,
"downloads": 0,
"stars": 0,
"created_at": "2026-07-31T00:00:00Z",
"updated_at": "2026-07-31T00:00:00Z"
}
}
```

Review the proposed catalog entry, the extension.yml manifest, README.md, and LICENSE from the linked extension repository. Start by locating the repository's extension catalog and its validation checks. Done means the Spec Switcher metadata is added accurately and the catalog still validates.

索引モデルが issue の本文から書いたものです。