Add `/security-review` command for automated vulnerability detection

### Describe the feature or problem you'd like to solve

GitHub Copilot CLI currently lacks a dedicated command for automated security reviews. Competing tools like Claude Code have implemented `/security-review` commands that help developers identify vulnerabilities before committing code. Given Microsoft and GitHub's commitment to security as a top priority, GitHub Copilot CLI should provide equivalent or superior security analysis capabilities.

### Proposed solution

Add a `/security-review` command that:
- Analyzes code for common vulnerabilities (SQL injection, XSS, authentication flaws, insecure data handling, dependency issues)
- Provides detailed explanations and remediation guidance
- Integrates seamlessly into the developer workflow
- Can be run on-demand before commits via CLI or as part of CI/CD pipelines via CLI injection in a GH action.

**Benefits:**
- Shifts security left by catching vulnerabilities during development
- Reduces security debt and remediation costs
- **Aligns with Microsoft's security-first principles** 👈🏽
- Maintains feature parity with competing AI coding assistants
- Leverages Copilot's existing code understanding capabilities

### Example prompts or workflows

1. **Pre-commit security check:**
```
> /security-review
Analyzing codebase for security vulnerabilities...
Found 3 potential issues in src/api/users.js
```

2. **Targeted file review:**
```
> /security-review src/auth/*.ts
Reviewing authentication modules...
⚠️ Weak password validation detected in src/auth/login.ts
```

3. **Dependency vulnerability scan:**
```
> /security-review --dependencies
Checking for known vulnerabilities in dependencies...
Critical: lodash@4.17.20 has prototype pollution vulnerability
```

4. **Configuration review:**
```
> /security-review --config
Analyzing security configurations...
ℹ️ CORS policy may be too permissive in config/server.js
```

5. **Post-fix verification:**
```
> /security-review src/db/queries.js
✅ No SQL injection vulnerabilities detected
```

### Additional context

- **Competitive landscape:** Claude Code already offers this feature, putting GitHub Copilot CLI at a disadvantage
- **Microsoft security commitment:** As part of Microsoft's Secure Future Initiative, developer tools should actively prevent security vulnerabilities. I mention this as a FTE of Microsoft and knowing this is shared responsibility between MSFT & GitHub.
- **Integration opportunity:** Could integrate with GitHub Advanced Security, Dependabot, and CodeQL for comprehensive analysis
- **User adoption:** Security-focused commands lower barriers to secure coding practices, especially for junior developers
- Claude Release announcement: [https://support.claude.com/en/articles/11932705-automated-security-reviews-in-claude-code](https://support.claude.com/en/articles/11932705-automated-security-reviews-in-claude-code)

Example: