fix(builtins): reject path traversal in patch diff headers

[chaliy]

Summary

• Add validate_path() that rejects paths containing .. components after strip_path()
• Prevents crafted diff headers like --- a/../../../etc/passwd from writing outside working directory
• Normal patch paths still work correctly

Test plan

• ../../../etc/passwd traversal rejected with error
• foo/../../secret.txt embedded traversal rejected
• Normal clean paths after strip still work
• All 20 patch unit tests pass
• All existing spec tests pass

Closes #989