chore: add .gitattributes to force Cargo.lock merge conflicts

Marks Cargo.lock as binary for merge purposes so git never silently
auto-merges it — forces regeneration on conflict instead.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: maxholman

.gitattributes

@@ -0,0 +1,3 @@
+# Force merge conflicts on lock files so they are always regenerated, never
+# silently auto-merged with potentially wrong dependency resolution.
+Cargo.lock merge=binary