fix: skip deploy-pr-preview for external repository PRs

Summary

Fixed the deploy-pr-preview workflow that was failing when PRs come from external repositories (forks) by adding a condition to skip the preview deployment job for external PRs.

Root cause: The website preview workflow attempts to deploy to the gh-pages branch, which requires write permissions that external contributors don't have, causing the workflow to fail with permission errors.

Solution: Added the condition if: github.event.pull_request.head.repo.full_name == github.repository to the preview job in .github/workflows/website-preview.yml. This ensures the preview deployment only runs for PRs from the same repository, not from forks.

Impact: External PRs will no longer fail the deploy-pr-preview workflow, while internal PRs will continue to generate preview deployments as expected.

Review & Testing Checklist for Human

• Verify YAML syntax is correct - Check that the added condition doesn't break the workflow syntax
• Test with external PR - Create or find a PR from a fork and verify the preview job is skipped
• Test with internal PR - Verify that PRs from the main repository still trigger preview deployments correctly

Diagram

%%{ init : { "theme" : "default" }}%%
graph TD
    PR["Pull Request Trigger"] --> PRC{{"PR Source Check<br/>head.repo.full_name == repository"}}
    
    PRC -->|"Internal PR<br/>(same repo)"| PREVIEW["preview job<br/>✅ Runs normally"]
    PRC -->|"External PR<br/>(fork)"| SKIP["preview job<br/>⚠️ Skipped"]
    
    PREVIEW --> CHECKOUT["Checkout code"]
    PREVIEW --> BUILD["Build website"] 
    PREVIEW --> DEPLOY["Deploy to gh-pages"]
    
    SKIP --> END["Workflow completes<br/>without errors"]
    DEPLOY --> END
    
    subgraph "Files"
        WPY[".github/workflows/website-preview.yml"]:::major-edit
    end
    
    subgraph Legend
        L1["Major Edit"]:::major-edit
        L2["Minor Edit"]:::minor-edit  
        L3["Context/No Edit"]:::context
    end

classDef major-edit fill:#90EE90
classDef minor-edit fill:#87CEEB  
classDef context fill:#FFFFFF

Notes

• This follows the standard GitHub Actions pattern for handling external PRs with write permissions
• The condition is added at the job level (more efficient than individual step conditions)
• Related to the GitHub Actions run failure: https://github.com/textlint/textlint/actions/runs/15951424181/workflow?pr=1604

Link to Devin run: https://app.devin.ai/sessions/69baed5652b6435eafa7ed650bdc266e
Requested by: @azu