feat: implement agentready submit CLI command

- Add PyGithub>=2.1.1 dependency for GitHub API integration
- Create submit.py with full PR creation workflow
- Validates GitHub token, assessment file, and repository access
- Generates unique ISO 8601 timestamp filenames
- Verifies submitter has commit access to repository
- Creates fork, branch, commits assessment, and opens PR
- Includes dry-run mode for testing
- Comprehensive error handling and user guidance

Submission workflow:
1. User runs: agentready submit
2. Command validates token and assessment
3. Verifies user has commit access to repo
4. Creates PR to agentready/agentready automatically
5. Validation workflow will run (next phase)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[email protected]>

Author: jeremyeder