Git helper subprocesses need timeouts and captured-output caps

[Widthdom]

Summary

Git helper commands drain stdout and stderr concurrently, but the child process has no timeout and captured output grows without bound. A hung git command, broken wrapper, pathological repository, or unexpectedly large output can hang cdidx or consume memory.

Evidence

• src/CodeIndex/Cli/GitHelper.cs:668 centralizes git subprocess capture.
• src/CodeIndex/Cli/GitHelper.cs:674 and src/CodeIndex/Cli/GitHelper.cs:675 append all stdout and stderr lines to StringBuilder without a cap.
• src/CodeIndex/Cli/GitHelper.cs:684 calls process.WaitForExit() with no timeout.

Expected

Git subprocess helpers should have a bounded runtime and bounded captured output, returning a clear diagnostic when either limit is hit.

[Widthdom]

Work started on branch fix-issue2832-2827.