chore: fix gitignore to exclude build artifacts in subdirectories

Changed /target/ and /_build/ to target/ and _build/ (without leading slash)
so they match build directories anywhere in the repo, not just at root level.

This properly ignores:
- impl/elixir/_build/
- impl/rust-cli/target/

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: Test

.gitignore

@@ -11,11 +11,11 @@ Thumbs.db
 .vscode/
 
 # Build
-/target/
-/_build/
-/build/
-/dist/
-/out/
+target/
+_build/
+build/
+dist/
+out/
 
 # Dependencies
 /node_modules/