fix(build): stop ignoring cmd/cyrene sources

The bare 'cyrene' .gitignore pattern matched the cmd/cyrene directory,
so trace_adapter.go was never committed and a fresh clone failed to
build (undefined: traceAdapter). Scope the pattern to the root binary
and add the missing file.

.gitignore

@@ -16,7 +16,7 @@ models.json
 *.dylib
 backend/cmd/*/main
 backend/cmd/*/cyrene
-cyrene
+/backend/cyrene
 
 # Go
 backend/vendor/