codeowners : use slash prefix for root files [no ci] (#16210)
This commit adds a leading slash to the paths of root-level files
in the CODEOWNERS file.
The motivation for this is that these might otherwise match files
in subdirectories that have other/additional owners will override them.
Refs: https://github.com/ggml-org/llama.cpp/pull/16209#issuecomment-3326434274
