fix: add missing sys import in watch.py

[shaun0927]

Fixes #386.

watch() references sys.platform at line 150 to select PollingObserver on macOS, but sys was never imported. This causes NameError on all platforms when running graphify watch.

Introduced in v0.4.15 by #373 (macOS PollingObserver change).

Change

One line: import sys added to module imports in graphify/watch.py.

Testing

# Before fix:
python -c "from graphify.watch import watch; from pathlib import Path; watch(Path('.'))"
# NameError: name 'sys' is not defined

# After fix:
python -c "from graphify.watch import watch; from pathlib import Path; watch(Path('.'))"
# [graphify watch] Watching /path - press Ctrl+C to stop

Full test suite: 433 passed, 0 failures.