Add exception in write-existing-file-guard for .sisyphus/*.md files
so Prometheus can rewrite plan files without being blocked by the guard.
The prometheus-md-only hook (which runs later) still validates that only
Prometheus can write to these paths, preserving security.
Fixes#1576
Adds a PreToolUse hook that intercepts write operations and throws an error
if the target file already exists, guiding users to use the edit tool instead.
- Throws error: 'File already exists. Use edit tool instead.'
- Hook is enabled by default, can be disabled via disabled_hooks
- Includes comprehensive test suite with BDD-style comments
YeonGyu-Kim