[Feature] Add GitHub cronjob support

[ali-sedaghatbaf]

Summary

• Adds GithubJobAnalyzeHandler (src/handlers/github_cronjob.py) mirroring the existing GitLab cronjob handler
• Exposes a new github-cronjob analyze --org-name <org> CLI command
• Adds GITHUB_TOKEN, GITHUB_USER_NAME, GITHUB_USER_EMAIL config vars
• Updates .env.sample, CLAUDE.md, and pyproject.toml (PyGithub==2.6.1)

Behavior

The handler iterates all source repos in a GitHub org, filters out archived repos, repos with stale commits, and repos that already have an open analysis branch or PR, then clones, runs the analyzer, and opens a PR — identical flow to the GitLab integration.

Test plan

• PYTHONPATH=src uv run python -c "from handlers.github_cronjob import GithubJobAnalyzeHandler" passes
• uv run src/main.py github-cronjob analyze --help shows correct args
• End-to-end run against a GitHub org where the token has write access creates a branch and opens a PR
• Re-running is a no-op (existing branch/PR detected and skipped)