Feature/GitHub pr data and agent summary

[xueyulinn]

Summary

Adds CodeHawky-powered PR summaries for GitHub pull request events, including full summaries for opened PRs and incremental summaries for synchronize events.

Related Issue

N/A

Type of Change

• Bug fix
• New feature
• Refactor
• Documentation
• Test
• CI/CD
• Chore

Changes Made

• Added a PR summary agent using the OpenAI Agents SDK with dedicated full and incremental prompt builders.
• Added a reviewer-focused system prompt that outputs Summary by CodeHawky with status, summary, risk, review path, and optional open questions.
• Added GitHub PR DTOs for pull request metadata, commits, changed files, opened webhook payloads, synchronize webhook payloads, and compare results.
• Added GitHub client support for fetching PR details, commits, changed files, compare results, and posting issue comments.
• Wired pull request opened events to generate full PR summaries.
• Wired pull request synchronize events to generate incremental summaries when the branch is ahead, and full regenerated summaries when the compare status is behind or diverged.
• Refactored GitHub installation token creation to accept explicit installation and repository identifiers.
• Removed the old generic GitHub DTO/webhook parsing modules in favor of DTO-based parsing.

How to Test

1. Run the test suite:

pytest

2. Trigger a GitHub pull_request.opened webhook and verify CodeHawky posts a full PR summary comment.
3. Trigger a GitHub pull_request.synchronize webhook where the branch is ahead and verify CodeHawky posts an incremental summary.
4. Trigger a synchronize event where the compare status is behind or diverged and verify CodeHawky posts a regenerated full PR summary with a reason.

Checklist

• Code builds successfully
• Tests added or updated
• Existing tests pass
• Documentation updated if needed
• No secrets or sensitive data included
• Breaking changes documented

Breaking Changes

None.