fix: path traversal security vulnerability in screenshot/pdf/eval commands

[Jah-yee]

Summary

Fixes security vulnerability reported in issue #13: Path traversal in screenshot/pdf/eval commands allows arbitrary file read/write.

Changes

• Add validateOutputPath() to ensure screenshot, pdf, and responsive commands only write to /tmp or current working directory
• Add validateReadPath() to prevent eval command from reading files outside allowed directories or using path traversal (..)
• Validates both absolute paths and relative paths with .. sequences

Security Impact

• Prevents arbitrary file write via screenshot/pdf/responsive commands
• Prevents arbitrary file read via eval command using path traversal

Testing

The fix adds path validation that rejects:

• Absolute paths outside /tmp or cwd
• Relative paths containing .. sequences

Example rejection messages:

Path must be within: /tmp, /home/user/project
Path traversal sequences (..) are not allowed

Fixes #13