Add repository instructions for coding agents

[greyli]

Summary

Add a root AGENTS.md so coding agents can find the repository layout, development setup, and validation commands before making changes. Document the shared and version-specific Bootstrap macros, existing compatibility paths, dependency generation, asset updates, and contribution practices.

This change only adds repository guidance; it does not implement any review findings or change runtime behavior.

Validation

• Checked setup instructions against docs/index.rst, and paths and test commands against tox.ini, .flake8, and the CI configuration.
• .venv/bin/python -m pytest -q: 167 passed, with 40 existing SQLAlchemy Query.get() deprecation warnings.
• git diff --cached --check: passed.
• Did not recreate the development environment or run the full tox matrix, lint, or Sphinx build for this Markdown-only change.