🔧 Add AGENTS.md and copilot-setup-steps workflow

[Copilot]

Mirror of executablebooks/markdown-it-py#380 for this repo.

• AGENTS.md: AI coding agent guide covering plugin architecture (block/inline/core rule patterns), development commands (tox), code style (Ruff/Mypy strict), fixture-based test patterns, and common operations (adding plugins, rules, renderers)
• .github/workflows/copilot-setup-steps.yml: Copilot environment setup — Python 3.11, uv, pre-commit, tox
• tox.ini: Added pre-commit and mypy tox environments so all commands referenced in AGENTS.md are available; fixed docs builder command to use {posargs} correctly (tox -e docs-update -- linkcheck instead of BUILDER=linkcheck)

Original prompt

Can you do similar to executablebooks/markdown-it-py#380 here

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.96%. Comparing base (d11bdaf) to head (a664c70).
⚠️ Report is 6 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #132      +/-   ##
==========================================
+ Coverage   92.80%   92.96%   +0.16%     
==========================================
  Files          31       32       +1     
  Lines        1835     1906      +71     
==========================================
+ Hits         1703     1772      +69     
- Misses        132      134       +2     

Flag	Coverage Δ
pytests	92.96% <ø> (+0.16%)	⬆️

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[chrisjsewell]

@copilot can you make sure that all the tox commands specified in the agents.md are actually available in the tox configuration, and if not add them (if applicable)

Also generally review the agents.md

[Copilot]

@copilot can you make sure that all the tox commands specified in the agents.md are actually available in the tox configuration, and if not add them (if applicable)

Also generally review the agents.m...

Audited all tox commands in AGENTS.md against tox.ini in a664c70:

1. Fixed incorrect command: BUILDER=linkcheck tox -e docs-update → tox -e docs-update -- linkcheck (tox.ini uses {posargs:html}, not a $BUILDER env var)
2. Added missing tox environments to tox.ini:
   • pre-commit — runs pre-commit run {posargs:--all-files}
   • mypy — runs mypy mdit_py_plugins {posargs}
3. Updated AGENTS.md Code Quality section with the new tox commands

Verified all environments work: tox -e mypy passes (32 source files, no issues), tox -e pre-commit passes (all hooks green), tox -e py310 passes (436 tests).