fix: write settings.json atomically in hooks.Install

[claude]

Summary

Replace the direct os.WriteFile in hooks.Install with an atomic temp-file + os.Rename pattern, consistent with how snapshot.go and activitylog.go already handle writes.

Before:

if err := os.WriteFile(settingsPath, finalJSON, 0600); err != nil {
    return nil, fmt.Errorf("writing settings.json: %w", err)
}

After:

tmp := settingsPath + ".tmp"
if err := os.WriteFile(tmp, finalJSON, 0600); err != nil {
    _ = os.Remove(tmp)
    return nil, fmt.Errorf("writing settings.json: %w", err)
}
if err := os.Rename(tmp, settingsPath); err != nil {
    _ = os.Remove(tmp)
    return nil, fmt.Errorf("writing settings.json: %w", err)
}

A mid-write crash now leaves the original settings.json intact rather than truncating/corrupting the user's entire Claude Code configuration.

Closes #256

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