feat(harbor-pilot): add Playwright test investigation directive

- Create comprehensive MDC rule for systematic Playwright test failure investigation
- Integrate rule into harbor_pilot_universal.mdc for team-wide access
- Include investigation workflow, diagnostic commands, and evidence-based analysis
- Document specific failure patterns (alert stacking, selector conflicts, timing issues)
- Provide practical examples from recent test failure investigation
- Add investigation commands for error context, trace files, and page snapshots

This rule transforms ad-hoc test debugging into systematic investigation process,
leveraging Playwright's built-in debugging tools for faster root cause identification.

.cursor/rules/harbor_pilot_universal.mdc

@@ -203,4 +203,5 @@ Follow this exact order **after** the Base Contract’s **Objective → Result
 - Respect Base *Do-Not* (no filler, no invented facts, no censorship).  
 - Prefer clarity over completeness when timeboxed; capture unknowns explicitly.
 - Apply historical comment management rules (see `.cursor/rules/historical_comment_management.mdc`)
-- Apply realistic time estimation rules (see `.cursor/rules/realistic_time_estimation.mdc`)
+- Apply realistic time estimation rules (see `.cursor/rules/realistic_time_estimation.mdc`)
+- Apply Playwright test investigation rules (see `.cursor/rules/playwright_test_investigation.mdc`)