Metrics sink should use private permissions and bounded rotation

[Widthdom]

Problem

src/CodeIndex/Cli/MetricsSink.cs opens the metrics JSONL file with FileMode.Append after creating the directory, but it does not force owner-only file permissions and does not enforce a maximum size or rotation policy.

Impact

Metrics can include command, path, timing, and failure details. On shared POSIX systems a permissive umask can expose the file, and long-running usage can grow it without bound.

Expected

Create metrics files with private permissions where supported, and add bounded rotation or a maximum-size policy similar to other long-lived logs.

[Widthdom]

I started work on this in branch fix-issue2843-2853-2854-2884.