🔧 Add AGENTS.md and copilot-setup-steps workflow (#132)

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`)

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: chrisjsewell <2997570+chrisjsewell@users.noreply.github.com>

Author: Copilot

.github/workflows/copilot-setup-steps.yml

@@ -0,0 +1,43 @@
+name: Copilot Setup Steps
+
+# Automatically run the setup steps when they are changed to allow for easy validation, and
+# allow manual testing through the repository's "Actions" tab
+on:
+  workflow_dispatch:
+  push:
+    paths:
+      - .github/workflows/copilot-setup-steps.yml
+  pull_request:
+    paths:
+      - .github/workflows/copilot-setup-steps.yml
+
+jobs:
+  # The job MUST be called `copilot-setup-steps` or it will not be picked up by Copilot.
+  copilot-setup-steps:
+    runs-on: ubuntu-latest
+
+    # Set the permissions to the lowest permissions possible needed for your steps.
+    # Copilot will be given its own token for its operations.
+    permissions:
+      contents: read
+
+    # You can define any steps you want, and they will run before the agent starts.
+    # If you do not check out your code, Copilot will do this for you.
+    steps:
+    - name: Checkout code
+      uses: actions/checkout@v4
+
+    - name: Set up Python
+      uses: actions/setup-python@v5
+      with:
+        python-version: "3.11"
+        cache: pip
+
+    - name: Install uv
+      run: pip install uv
+
+    - name: Install pre-commit and pre-commit-uv
+      run: pip install pre-commit pre-commit-uv
+
+    - name: Install tox and tox-uv
+      run: pip install tox tox-uv