Feature: support for `git grep` command

Summary

git grep is not intercepted by RTK, so its output is passed through unfiltered to the model context.

Usage data

From rtk discover on an Android monorepo project (last 30 days):

• 67 invocations of git grep not handled by RTK

Example commands seen:

git grep -i -e "android-cli" -e "android" --
git grep -r "SomeClass" -- "*.kt"
git grep -l "import com.bumptech.glide"

Why it matters

git grep searches across the entire git tree and can return thousands of matching lines across hundreds of files. Without filtering, the full output lands in the model context. RTK's existing grep handler already handles grep -r — git grep deserves the same treatment.

Expected behavior

rtk git grep <args> should apply the same line-limiting / truncation strategy as rtk grep, surfacing the most relevant matches and summarizing the rest.

Current workaround

None — git grep bypasses the hook entirely since it's a git subcommand, not a standalone grep invocation.