fix: remove dead try/catch in insertNode; fix SENSITIVE_PATHS case-sensitivity (#327)

Drop the no-op try/catch around insertNode.run, and lowercase the Windows
SENSITIVE_PATHS entries so validateProjectPath's case-insensitive check
actually blocks c:\windows. Adds a validateProjectPath test (POSIX +
Windows-gated); the Windows-gated case was validated on a real Windows 11 VM.

Closes #327