The repo had no root-level .gitignore, so .dart_tool/, ephemeral plugin
symlinks and build artifacts were being tracked across the workspace.
Every flutter pub get regenerated them and they polluted PRs as recurring
noise.
- Add root .gitignore covering .dart_tool/, ephemeral plugin symlinks,
build/, coverage/, IDE files
- Untrack ~39 existing cache files across root, modules and packages