feat: Add Playwright testing guide

[falfaddaghi]

Summary

Adds a Nexus-specific Playwright testing skill and companion guide so browser-testing agents can navigate the app, manage auth/session state, and exercise core workflows with less guesswork. The documentation covers route landmarks, optional authentication behavior, screenshot conventions, editor/workspace smoke tests, and troubleshooting while keeping runtime app behavior unchanged.

Plan: docs/tasks/playwright-testing-skill-0242b1d9/plan-playwright-testing-skill-0242b1d9.md

ADW tracking ID: 0242b1d9

Checklist

• Verified current route and optional auth behavior from the app sources
• Added .claude/skills/nexus-playwright/SKILL.md with project-specific playwright-cli quick-start guidance
• Added docs/testing/playwright-agent-guide.md with route map, auth/session handling, screenshots, workflows, and troubleshooting
• Linked the browser testing guide from CONTRIBUTING.md
• Kept the change documentation-only with no runtime app behavior changes

Key changes

• Introduces a reusable local agent skill for Nexus browser testing.
• Documents expected landing, editor, workspace, auth, OpenCode, persistence, and screenshot flows.
• Ignores local Playwright screenshot/auth artifacts to keep test runs clean.