fix(test-browser): detect dev server port from project config

[mvanhorn]

Summary

• Add new step 4 (Detect Dev Server Port) with cascading port detection:
  1. Explicit --port argument (e.g., /test-browser --port 5000)
  2. CLAUDE.md / project instructions (regex for port, PORT, localhost:\d+)
  3. package.json dev scripts (--port flags)
  4. Environment files (.env, .env.local, .env.development)
  5. Falls back to 3000 if nothing found
• Replace all hardcoded localhost:3000 with localhost:${PORT} using detected port
• Update "Server not running" error to show detected port and suggest --port flag
• Renumber subsequent steps (5-8 become 6-9) to accommodate the new step

Closes #164

Test plan

• Run /test-browser in a project with default port 3000 - auto-detects and works as before
• Run /test-browser --port 5000 - uses port 5000
• Add PORT=4000 to .env and run /test-browser - detects port 4000
• Add localhost:8080 reference in CLAUDE.md - detects port 8080
• Verify no remaining hardcoded localhost:3000 in the file

Co-Authored-By: Claude Opus 4.6 noreply@anthropic.com