fix: guard POSIX hook commands

[ousamabenyounes]

Fixes #646.
Fixes #645.

What changed

• Normalize backslashed POSIX plugin roots before invoking hook scripts.
• Guard POSIX hook commands when node is missing or the script path is unavailable.
• Apply the same POSIX hardening to Copilot bash hooks.
• Keep Windows-specific commands covered by commandWindows/powershell.

RED

node --test tests/hooks.test.js

Error: Cannot find module .../\home\...\ponytail/hooks/ponytail-activate.js
Tests: 1, Failures: 1

GREEN

node --test tests/hooks.test.js tests/hooks-windows.test.js
# tests 7
# pass 7

npm test --prefix pi-extension
# tests 23
# pass 23

npm test --prefix ponytail-mcp
# tests 3
# pass 3

Full-suite note

node --test tests/*.test.js
# tests 82
# pass 81
# fail 1

The remaining failure is csv: correct pandas one-liner passes; this environment has no pandas installed (ModuleNotFoundError: No module named pandas) and the failure is unrelated to the hook manifest changes.