docs: clarify Python repository and package structure

[rlippmann]

Summary

• make the root README the canonical guide for this Python-only repository
• move installation, reference-integration navigation, and run guidance to the root README
• reduce python/README.md to package/PyPI-facing installation and contents guidance
• use a PyPI-safe absolute root README link and an unambiguous package title
• remove obsolete cross-language and language-selection wording from AGENTS.md
• bump the Python package version to 0.2.1 and refresh the matching lockfile metadata

Validation

• npx --yes markdownlint-cli2 — passed, 0 issues
• git diff --check — passed
• uv lock --check — passed
• confirmed no source or test changes
• dependency ranges and package dependencies unchanged