uv sync fails on Intel macOS: onnxruntime 1.29.0 has no x86_64 wheel (blocks Chapter 5)

## Summary
On Intel macOS, `uv sync` hard-fails resolving `onnxruntime==1.29.0` (pulled in transitively by
`chromadb`, used for the Chapter 5 knowledge base). That version has no macOS x86_64 wheel.

## Error
```
error: Distribution `onnxruntime==1.29.0` can't be installed because it doesn't have a source
distribution or wheel for the current platform
hint: You're on macOS (macosx_x86_64), but onnxruntime (v1.29.0) only has wheels for
manylinux..., macosx_14_0_arm64, win_...
```

## Environment
macOS Intel (x86_64), Python 3.13.5 (uv-managed), current main.

## Workaround that works
Installing with `uv pip install chromadb PyPDF2 ...` (instead of `uv sync`) resolves
`onnxruntime==1.23.2`, which does have an Intel-mac wheel, and Chapter 5 then builds the ChromaDB
knowledge base fine.

## Possible fixes (maintainers' call)
- Add a `tool.uv.required-environments` entry so uv resolves a wheel-compatible onnxruntime on
x86_64 macOS, or
- Note the limitation + workaround in the Chapter 5 / setup docs, or
- Constrain chromadb/onnxruntime to a version with x86_64 macOS wheels.

Apple Silicon and Linux are unaffected. Happy to open a PR once you decide the preferred approach.

Start by reproducing the failure with `uv sync` on Intel macOS and inspect the dependency resolution for `chromadb` and `onnxruntime==1.29.0`. Review the proposed `tool.uv.required-environments` configuration and the Chapter 5/setup documentation before choosing the maintainer-approved approach. Done means the chosen fix is documented or implemented, `uv sync` succeeds on x86_64 macOS, and Chapter 5 builds the ChromaDB knowledge base.

Written by the indexing model from the issue text.