Bound worktree .git and commondir reads during repository resolution

[Widthdom]

Problem

src/CodeIndex/Cli/GitHelper.cs resolves linked worktrees by reading .git and commondir files with File.ReadAllText and no size check.

Impact

A malformed checkout or attacker-controlled workspace can make repository resolution allocate large buffers before normal git safeguards run.

Expected

Treat these metadata files as tiny control files: check length before reading, reject oversized content, and keep the existing linked-worktree behavior for valid files.

[Widthdom]

Work started on branch fix-issue2844-2856-2857-2870-2858-2871-2872-2867.