chore: Remove plan file from repo and add plans/ to .gitignore

- Move PLAN_PR1618_REIMPLEMENTATION.md to local plans/ folder
- Add plans/ to .gitignore to keep plans local

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: cfsmp3

.gitignore

@@ -143,6 +143,9 @@ bazel*
 #Intellij IDEs
 .idea/
 
+# Plans (local only)
+plans/
+
 # Rust build and MakeFiles (and CMake files)
 src/rust/CMakeFiles/
 src/rust/CMakeCache.txt