Implement GitHub webhook sync and repo sync logic

[kivo360]

Problem

The GitHub integration routes have incomplete sync logic:

• backend/omoi_os/api/routes/github.py line ~156: Webhook secret hardcoded to None instead of being read from project settings
• backend/omoi_os/api/routes/github.py line ~206: TODO: Implement sync logic — the GitHub repo sync endpoint is a stub
• backend/omoi_os/api/routes/github_repos.py line ~323: TODO: Check if webhook is configured — always returns false

What to do

1. Load webhook secret from project settings or config
2. Implement the sync logic — pull latest commits, branches, PRs from GitHub API
3. Wire up webhook configuration check to return actual status

The GitHubIntegrationService and GitHubApiService already handle most GitHub API interactions.

Difficulty: Medium | Skill: Python (FastAPI, GitHub API)