Feature: expose a programmatic Python API

[b-j-karl]

Currently api-squash is CLI-only. A public Python API would allow programmatic use:

```python
from api_squash import summarize_project, summarize_file

result = summarize_project("src/", no_docstrings=True)
```

The internals already support this — `extractor.py`, `renderer.py`, and `scanner.py` do the work. This is a matter of defining the public API surface, stabilizing it, and documenting it.

This also unblocks:

• MCP server integration (call api-squash from an MCP tool handler)
• GitHub Action (invoke from Python in a workflow)
• Embedding in other tools