chore(ai): Setup minimal configuration (issue #8)

.ai/AGENTS.md

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+# Padi PHP Project Guidelines
+
+A personal library for implementing design patterns and utilities. All development happens in Docker containers for consistency.
+
+## Code Style
+
+**PSR-12 with Custom Extensions** (enforced by Mago):
+- Single quote for code string, like technical name, id, etc and double quotes for strings, like error message, user output, etc
+
+**Immutability & Type Safety**:
+- Comprehensive type hints on parameters and returns
+
+See [mago.toml](mago.toml) for exact linting rules and analysis thresholds.
+
+## Architecture
+
+**Design Patterns Library**: Flat structure demonstrating clean code principles
+- `src/` — Main classes and utilities (PSR-4 namespace: `Padi\`)
+- `tests/` — PHPUnit tests mirroring source structure
+- Value Objects: Immutable, validated (Email, etc.)
+
+## Commands
+
+**Prerequisites**: Docker + Make
+
+All commands execute inside Docker containers. See [Makefile](Makefile) for full options.
+
+### Build project
+
+```bash
+make composer-install # Build Docker image & install dependencies (run first!)
+```
+
+### Run tests
+
+```bash
+make test                      # Run all PHPUnit tests
+make test ARGS="path/to/test"  # Run specific test file
+```
+
+### Format code, lint & analysis issue
+
+```bash
+make fix
+```
+
+## Conventions
+
+**Testing**:
+- PHPUnit 13.x for unit tests
+- Test files in `tests/` named `{ClassName}Test.php`
+- Extend `PHPUnit\Framework\TestCase`
+- Test exception behavior explicitly
+
+**Method Naming**:
+- CamelCase for classes/methods
+- UPPER_SNAKE_CASE for constants
+
+**File Organization**:
+- One class per file (PHP standard)
+
+**Development Notes**:
+- First run: Always execute `make composer-install` to build the Docker image
+- Permissions: UID/GID are passed to containers to prevent file permission issues
+- `.image-built` marks successful image build — delete to rebuild
+- vendor/ and .image-built are git-ignored

.github/copilot-instructions.md

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+# Padi PHP — AI Guidelines
+
+See [.ai/AGENTS.md](../.ai/AGENTS.md) for workspace instructions and conventions.