Test PR comment functionality

[imjasonh]

Summary

This PR tests the apidiff-action's ability to comment on pull requests when breaking API changes are detected.

Changes

• Add pull-requests: write permission to CI workflow
• Add new test-pr-comment job that runs only on PRs
• Enable comment-on-pr: true for the breaking changes test

Expected Result

When this PR is merged, the action should automatically add a comment showing:

# API Compatibility Check Results

## Summary

| Type | Count |
|------|-------|
| Breaking changes | 3 |
| Compatible changes | 1 |

⚠️ **This PR contains breaking API changes\!**

## Details

#### ❌ Breaking changes

- (*Greeter).Greet: changed from func() string to func(bool) string
- MaxRetries: removed
- Process: changed from func(string) (string, error) to func(context.Context, string) (string, error)

#### ✅ Compatible changes

- Greeter.Language: added

This demonstrates the action working on real PRs to help reviewers understand API changes.

🤖 Generated with Claude Code

[github-actions]

API Compatibility Check Results

Summary

Type	Count
Breaking changes	3
Compatible changes	1

⚠️ This PR contains breaking API changes!

Details

❌ Breaking changes

• (*Greeter).Greet: changed from func() string to func(bool) string
• MaxRetries: removed
• Process: changed from func(string) (string, error) to func(context.Context, string) (string, error)

✅ Compatible changes

• Greeter.Language: added