Unanchored `build/` in .gitignore matched `src/model/encoder/model/build/`,
causing three source files to go untracked and breaking CI compilation
(`mod build_bert`, `mod build_gemma`, `mod build_nomic` had no backing files
on the remote). Changed to `/build/` so only the top-level build artifact
directory is ignored.

Adds the three previously-ignored source files:
- src/model/encoder/model/build/build_bert.rs
- src/model/encoder/model/build/build_gemma.rs
- src/model/encoder/model/build/build_nomic.rs