🔧 Add agent setup files with tox and dependency groups

[chrisjsewell]

This PR adds comprehensive tooling and documentation to enable GitHub Copilot agents (and other AI coding assistants) to work effectively with the sphinx-codelinks project.

Summary

Add agent documentation, GitHub Copilot setup workflow, tox configuration for standardized development, and PEP 735 dependency groups for modern dependency management.

Inspiration

This PR follows patterns established in these repositories:

• useblocks/sphinx-needs — AGENTS.md structure, dependency groups
• executablebooks/sphinx-design — tox.ini, copilot-setup-steps.yml, commit message format
• executablebooks/MyST-Parser — AGENTS.md sections, Sphinx integration documentation

Changes

New Files

File	Description
AGENTS.md	Comprehensive guide for AI coding agents (~450 lines) with project overview, repository structure, development commands, code style guidelines, commit message format, testing guidelines, Sphinx integration flow diagram, architecture documentation, and common patterns

Modified Files

File	Changes
pyproject.toml	Added [dependency-groups] section with testing, docs, mypy, ruff, and build groups (PEP 735)

Details

AGENTS.md highlights

• Project overview and repository structure
• Development commands (all tox-based)
• Code style guidelines with markdownlint, Ruff, and Mypy
• Commit message format with emoji keywords (✨, 🐛, 📚, etc.)
• Sphinx integration flow with mermaid diagram and event handler table
• Common patterns for adding languages, markers, CLI commands, and config options
• Reference documentation with links to all dependencies

tox.ini environments

• py{312,313,314}-sphinx{7,8,9} — Test matrix across Python and Sphinx versions
• docs — Documentation building with CLEAN/BUILDER env var support
• mypy — Type checking
• ruff-check / ruff-fmt — Linting and formatting

pyproject.toml dependency groups

• testing — pytest, syrupy, coverage tools
• docs — Sphinx themes and documentation extensions
• mypy — Type checking and stub packages
• ruff — Linting/formatting
• build — Build and packaging tools

[ubmarco]

tox fails for me locally

py314-sphinx7: Exception running subprocess [Errno 2] No such file or directory: 'pytest'
py314-sphinx7: exit 2 (0.00 seconds) /home/marco/git/useblocks/sphinx-test-reports> pytest tests
py314-sphinx7: FAIL ✖ in 16.04 seconds

[chrisjsewell]

@ubmarco yeh needs to look more like this https://github.com/executablebooks/MyST-Parser/blob/master/tox.ini

Also the ### Testing of AGENTS.md needs to be updated, to use the correct environments

[chrisjsewell]

@ubmarco fixed in d302a7c