Fix editable package layout

[rlippmann]

Summary

Fix the Python package layout so context_compiler_example_integrations is importable during Hatch editable installs while preserving the existing installed-package import paths and built wheel contents.

The change adds a real source-tree package root and replaces the unsupported Hatch source-prefix rewrite with explicit force-include mappings for the existing Python examples, reference integrations, and README.

Why

The packaging layout introduced for 0.1.0 produced correct wheel contents but failed during normal editable development installs:

uv sync --group dev

Hatch does not support editable installations when a sources path rewrite changes a prefix rather than removes it. As a result, clean development environments could not install the repository and ./scripts/validate_python.sh could not run.

This keeps the intended installed package layout while making editable installs work correctly.

Example metadata

• Enforcement point: N/A
• Domain: Repository packaging
• Technology/runtime: Python / Hatch / uv
• Example type: repository maintenance
• Does this use directive drafter? no
• Does this derive state from model output? no

Checklist

• Repository scaffolding / governance change
• One primary enforcement point
• Explicit authoritative state
• No model-derived state mutation
• Adversarial stub or equivalent test
• Observable runtime behavior change
• Domain vocabulary is natural
• Framework is secondary to the enforcement point
• Docs updated
• Tests/smoke checks added where practical

Notes

Validation:

• uv sync --group dev — passed
• ./scripts/validate_python.sh — passed (201 passed, 6 skipped)
• uv build — passed
• context_compiler_example_integrations import verified in editable mode
• built wheel contents verified to preserve the expected installed package layout

No runtime behavior or example semantics are changed.