feat!: migrate to async-only architecture for v2.0.0

BREAKING CHANGE: Complete removal of synchronous API in favor of async-only implementation

- Remove all sync implementation files (client.py, order_manager.py, etc.)
- Remove all sync examples
- Remove all sync test files
- Update __init__.py to export async classes with original names
- Update all examples to use new import names (ProjectX instead of AsyncProjectX)
- Update factory functions to be async-only
- Update README.md with async-first documentation and migration guide
- Set version to 2.0.0

This is a major breaking change. All APIs now require async/await patterns.
See README.md for migration guide from v1.x to v2.0.0.

Author: TexasCoding