Replace brittle string checks with robust path.resolve/relative validation:
- Fix Windows backslash paths (.sisyphus\plans\x.md) being incorrectly blocked
- Fix case-sensitive extension check (.MD now accepted)
- Add workspace confinement (block paths outside root even if containing .sisyphus)
- Block nested .sisyphus directories (only first segment allowed)
- Block path traversal attempts (.sisyphus/../secrets.md)
- Use ALLOWED_EXTENSIONS and ALLOWED_PATH_PREFIX constants (case-insensitive)
The new isAllowedFile() uses Node's path module for cross-platform compatibility
instead of string includes/endsWith which failed on Windows separators.
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