Detect externally modified items in the GUI

[buzzkill58]

Summary

• reload the selected item from disk before displaying it
• fingerprint the selected YAML file and prevent a stale GUI edit from overwriting a newer external change
• show a warning when the external version wins
• add focused tests for content changes, missing files, and blocked stale saves

Testing

• python -m pytest -q doorstop/gui/tests/test_application.py: 4 passed
• manually verified on Windows with Python 3.14 that reselecting an item loads an external YAML update
• manually verified that a stale GUI edit shows the warning, is discarded, and preserves the external YAML content