docs(core): expand README API reference

[ulises-jeremias]

Summary

• Expand packages/create-python-app-core/README.md to parity with @create-node-app/core documentation
• Add overview, installation, full exported API table, CPA_* environment variables, module map, usage examples, and error codes

Closes#172

Test plan

• README renders correctly (ASCII-only, no broken tables)
• API table matches create_python_app_core.__all__ exports
• Environment variables and error codes match source (paths.py, git_cache.py, errors.py, installer.py)
• Usage examples reflect actual function signatures from api.py, installer.py, config.py, paths.py, git_cache.py

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