fix: package core runtime only

[rlippmann]

What changed

• Removed examples/, demos/, and host_support/ from the Python wheel.
• Kept example and demo source files in the repository and source distribution.
• Removed the unused examples extra.
• Retained the demos extra as a dependency bundle for running repository demos.
• Updated packaging and installation documentation to clarify the distinction between runtime installs and repository artifacts.
• Bumped the Python package version to 0.8.1.

Why

• Extras control dependencies, not which files are included in a wheel.
• The 0.8.0 wheel unintentionally shipped repository artifacts that are not part of the core runtime contract.
• The intended packaging model is:
  • pip install context-compiler → core runtime only
  • repository checkout → examples, demos, and integration artifacts
  • context-compiler[demos] → optional dependencies for running repository demo scripts

Checklist

• pre-commit run (uv run pre-commit run --all-files)
• tests pass (uv run pytest)