[Extension]: Add markdown-linter

### Extension ID

markdown-linter

### Extension Name

Markdown Linter

### Version

1.0.0

### Description

Validates markdown files in spec documents for formatting consistency, broken links, and style guide compliance

### Author

Devdeep781

### Repository URL

https://github.com/Devdeep781/spec-kit-test-extension

### Download URL

https://github.com/Devdeep781/spec-kit-test-extension/archive/refs/tags/v1.0.0.zip

### License

MIT

### Homepage (optional)

_No response_

### Documentation URL (optional)

_No response_

### Changelog URL (optional)

_No response_

### Required Spec Kit Version

>=0.1.0

### Required Tools (optional)

```markdown

```

### Number of Commands

2

### Number of Hooks (optional)

_No response_

### Tags

docs, linting, markdown, validation

### Key Features

- Lint markdown files for formatting issues
- Check for broken links in documentation
- Auto-fix common formatting problems
- Configurable rules via speckit.yml

### 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:
- Ubuntu 22.04 with Python 3.11
- macOS 14.0

Test project: Personal spec-kit project with 50+ markdown files

### Example Usage

```markdown
specify extension add markdown-linter --from https://github.com/Devdeep781/spec-kit-test-extension/archive/refs/tags/v1.0.0.zip
speckit markdown-linter lint --fix
```

### Proposed Catalog Entry

```json
{
"markdown-linter": {
"name": "Markdown Linter",
"id": "markdown-linter",
"description": "Validates markdown files in spec documents for formatting consistency",
"author": "Devdeep781",
"version": "1.0.0",
"download_url": "https://github.com/Devdeep781/spec-kit-test-extension/archive/refs/tags/v1.0.0.zip",
"repository": "https://github.com/Devdeep781/spec-kit-test-extension",
"homepage": "https://github.com/Devdeep781/spec-kit-test-extension",
"license": "MIT",
"requires": {
"speckit_version": ">=0.1.0"
},
"provides": {
"commands": 2,
"hooks": 0
},
"tags": ["docs", "linting", "markdown", "validation"],
"verified": false,
"downloads": 0,
"stars": 0,
"created_at": "2026-08-10T00:00:00Z",
"updated_at": "2026-08-10T00:00:00Z"
}
}
```

### Additional Context

_No response_