feat: add PW_SCRIPT_DIR support to persist test scripts

[alexanderbailey]

Summary

• Adds PW_SCRIPT_DIR environment variable support to run.js — when set, executed scripts are automatically copied to that directory
• Directory is created automatically if it doesn't exist; duplicate filenames get a timestamp suffix to avoid overwrites
• Updates SKILL.md with a new "Persisting Test Scripts" section, workflow examples, and a tip reminding Claude to suggest the option when users ask to save/reuse tests

Test plan

• Run a script without PW_SCRIPT_DIR set — confirm existing behaviour unchanged
• Run a script with PW_SCRIPT_DIR=./playwright-tests — confirm script is copied and 💾 Script saved to: is printed
• Run the same script again — confirm timestamp suffix is added to avoid overwrite
• Run with inline code or stdin — confirm no save attempt is made
• Set PW_SCRIPT_DIR to a non-existent path — confirm directory is created automatically

Closes #15