Feat: Implement PR Syncer for GitHub Workflow

[tobiu]

As the final part of the sync service refactoring epic, this ticket introduces the new functionality to sync GitHub Pull Requests to the local filesystem.

Tasks:

1. Create ai/mcp/server/github-workflow/services/sync/PrSyncer.mjs.
2. Implement the logic for a 1-way pull sync of PRs. This will involve:
   • Fetching PR data via the GraphQL API.
   • Formatting the PR content (including metadata, description, and potentially comments) into a structured Markdown file.
   • Saving the files to a new local directory (e.g., .github/PULL_REQUESTS/).
3. Integrate the new PrSyncer into the main SyncService orchestration loop.
4. Update the MetadataManager to handle metadata for synced PRs (e.g., storing updatedAt and contentHash).