[M4] Playwright Test Runner Integration

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Milestone 4 — Playwright Test Video Integration | Priority: High | COMPLETED in PR #27

Run existing Playwright test suites with video+tracing enabled, collect video and trace artifacts.

• Supports pytest-playwright and @playwright/test frameworks
• Auto-collects video + trace from output dirs
• Graceful failure handling (retain_on_failure)
• CLI: docgen playwright-test [--test filter] [--timeout 300]

Implemented in src/docgen/playwright_test_runner.py. See issues/playwright-test-integration/02-test-runner-integration.md for full spec.