Add AGENTS.md (#494)

Add condensed agent instructions in AGENTS.md with:
- Quick reference for common commands
- Project structure overview
- Code style patterns with examples
- Cookiecutter workflow summary

Author: llucax

AGENTS.md

@@ -0,0 +1,99 @@
+# AGENTS.md - AI Coding Agent Instructions
+
+Condensed instructions for AI agents. See [README.md](README.md) and [CONTRIBUTING.md](CONTRIBUTING.md) for full details.
+
+## Project Overview
+
+Python package (3.11+) providing repository configuration and scaffolding for Frequenz projects: cookiecutter templates (actor, api, app, lib, model), nox sessions, protobuf/gRPC utilities, MkDocs and pytest utilities.
+
+**Package:** `src/frequenz/repo/config/` (namespace package, `setuptools_scm` versioning)
+
+## Quick Reference
+
+```sh
+# Setup
+pip install -e .[dev]              # All dev dependencies
+pip install -e .[dev-noxfile]      # Just noxfile deps
+
+# Test
+nox -s pytest_max                  # Full test suite
+nox -R -s pytest_max               # Reuse venv (faster)
+pytest tests/                      # Direct pytest
+UPDATE_GOLDEN=1 pytest tests/integration/test_cookiecutter_generation.py::test_golden
+
+# Lint (run before committing)
+nox                                # All checks
+nox -s formatting                  # black + isort
+nox -s mypy                        # Type checking
+nox -R -s mypy                     # Reuse venv
+```
+
+**Markers:** `integration`, `cookiecutter`
+
+## Project Structure
+
+```
+src/frequenz/repo/config/       # Main package (_core.py, nox/, mkdocs/, pytest/, setuptools/, cli/)
+tests/                          # Unit tests, tests/integration/ for integration tests
+tests_golden/                   # Golden test fixtures
+cookiecutter/                   # Cookiecutter templates
+```
+
+## Code Patterns
+
+```python
+# File header (required)
+# License: MIT
+# Copyright © 2023 Frequenz Energy-as-a-Service GmbH
+"""Module docstring."""
+
+import dataclasses
+from typing import Self, assert_never
+
+# Dataclasses: always kw_only + frozen if intended to be immutable
+@dataclasses.dataclass(kw_only=True, frozen=True)
+class Config:
+    """Docstring."""
+    opts: CommandsOptions = dataclasses.field(default_factory=CommandsOptions)
+
+# Functions: strict typing, Google docstrings
+def func(session: _nox.Session, /, *, flag: bool = True) -> list[str]:
+    """Brief description.
+
+    Args:
+        session: The nox session.
+        flag: Optional flag.
+
+    Returns:
+        List of strings.
+    """
+```
+
+| Type | Convention | Example |
+|------|------------|---------|
+| Files/functions | snake_case | `api_pages.py`, `find_dirs()` |
+| Classes | PascalCase | `RepositoryType` |
+| Constants | UPPER_SNAKE | `UPDATE_GOLDEN` |
+| Private | `_` prefix | `_impl()` |
+
+**Formatting:** black formatting using 4 spaces (Python), 2 spaces (YAML/TOML/JSON), 88 line length, 100 max, double quotes
+
+When changing files that are regularly reset (like `RELEASE_NOTES.md` or `cookiecutter/migrate.py`), consult the git history to match the style used in the past.
+
+## Commit Messages
+- Use imperative mood: "Add", "Fix", "Update"
+- Use a 50-character limit for the subject line (can go a bit over if necessary), and wrap body at 80 characters
+- Include a brief description of the change in the body if needed, focus on the what and why, and avoid describing how, especially if it is obvious from the code/diff
+- Separate changes into logical commits
+- Consult the recent git history, specifically of the files being committed, to match style and conventions in commit messages used previously.
+
+## Cookiecutter Template Changes
+
+See [CONTRIBUTING.md](CONTRIBUTING.md#modifying-cookiecutter-templates) for the full workflow. Summary:
+
+1. Edit templates in `cookiecutter/{{cookiecutter.github_repo_name}}/`
+2. Update golden files: `UPDATE_GOLDEN=1 pytest tests/integration/test_cookiecutter_generation.py::test_golden`
+3. Write migration in `cookiecutter/migrate.py` (idempotent; use `manual_step()` for non-automatable changes)
+4. Validate: `python3 cookiecutter/migrate.py && git diff .github/`
+5. Update `RELEASE_NOTES.md`
+6. Commit separately: templates first, then this repo's migration result