docs: add TDD conventions and testing guide to CLAUDE.md

Covers testing file conventions, Red→Green→Refactor cycle with
example prompts, common anti-patterns, test runner commands, and
the auto-run hook setup.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -16,6 +16,106 @@ curl "http://localhost:8080/graph.svg?theme=dark" -o graph.svg
 curl "http://localhost:8080/health"
 ```
 
+### Testing conventions (apply to every test file)
+
+- `declare(strict_types=1)` at the top of every test file
+- Test classes are `final` and extend `TestCase`
+- Methods: `#[Test]` attribute + `it_` prefix + `snake_case` — e.g. `it_returns_empty_when_no_contributions`
+- Structure: Arrange → Act → Assert, separated by blank lines; one assertion per test when possible
+- Use `$this->assert*()` not `self::assert*()`
+
+```php
+<?php
+
+declare(strict_types=1);
+
+namespace App\Tests\Service;
+
+use App\Service\SvgRenderer;
+use PHPUnit\Framework\Attributes\Test;
+use PHPUnit\Framework\TestCase;
+
+final class SvgRendererTest extends TestCase
+{
+    #[Test]
+    public function it_renders_an_svg_with_no_contributions(): void
+    {
+        $renderer = new SvgRenderer();
+
+        $svg = $renderer->render([], 'light');
+
+        $this->assertStringContainsString('<svg', $svg);
+    }
+}
+```
+
+### TDD: Red → Green → Refactor
+
+Always drive new features and bug fixes with this cycle. Claude writes implementation first by default — override that with explicit prompts.
+
+**Phase 1 — Red (write a failing test first)**
+
+```
+Write a failing test for [feature description].
+Do NOT write the implementation yet.
+The test should fail because the class/method does not exist.
+```
+
+**Phase 2 — Green (minimal implementation)**
+
+```
+The tests are written and failing. Now implement the minimum code to make them pass. Nothing more.
+```
+
+**Phase 3 — Refactor**
+
+```
+Tests are green. Refactor the implementation for [readability / removing duplication / naming].
+Run php bin/phpunit after each change to confirm tests stay green.
+```
+
+**Common anti-patterns**
+
+| Wrong prompt | Why it breaks TDD | Correct prompt |
+|---|---|---|
+| "Write tests for this feature" | Claude implements first, then fits tests to it | "Write **failing** tests for [feature]. Stop before any implementation." |
+| "Add tests and implementation" | Loses the design feedback of failing tests | Two separate prompts: Red, then Green |
+| "Make the tests pass" | Encourages skipping to a green state | "Implement the minimum to make the failing tests pass." |
+| Combining Red + Green in one request | No failing baseline | Always separate the two phases |
+
+### Running tests
+
+```bash
+# Run full suite
+php bin/phpunit
+
+# Run with human-readable output
+php bin/phpunit --testdox
+
+# Run a single test file
+php bin/phpunit tests/Unit/Service/SvgRendererTest.php
+
+# Run tests matching a filter
+php bin/phpunit --filter it_renders
+```
+
+### Auto-run hook
+
+Add to `.claude/settings.json` to run PHPUnit automatically after every file edit:
+
+```json
+{
+  "hooks": {
+    "PostToolUse": [
+      {
+        "matcher": "Edit|Write",
+        "command": "php bin/phpunit 2>&1 | tail -20"
+      }
+    ]
+  }
+}
+```
+
 ## Docker
 
 ```bash