Abstract git remote sync to support GitHub, GitLab, Gitea, and other providers

[JohnRDOrazio]

Summary

The current remote sync implementation is GitHub-specific (github_service.py). The git operations via pygit2 are already provider-agnostic, but the API layer for PR sync, webhook handling, and repository operations only supports GitHub.

This should be abstracted into a provider-agnostic interface so that OntoKit can sync with any git-compatible remote: GitHub, GitLab, Gitea, Bitbucket, or any self-hosted git server.

What needs to change

• Define a RemoteProvider interface with methods for:
  • PR lifecycle: create, update, merge, close, reopen, list
  • Reviews: create, list
  • Comments: create, list
  • Webhooks: setup, verify signature, handle events
  • Repository info: scan files, get content, get repo metadata
• Refactor github_service.py into a GitHubProvider implementation of that interface
• Add implementations for at least GitLab and Gitea
• Update GitHubIntegration model to a more generic RemoteIntegration model with a provider field
• Update webhook handler to route events based on provider type (different payload formats)
• Update github_sync.py — the git-level push/pull is already provider-agnostic (just needs the remote URL), but any API calls need to go through the provider interface

Context

Raised in discussion on #13. OntoKit should be deployable without any dependency on a specific commercial git hosting service. The sync provider must be interchangeable and optional.