fix: actually include cmd/service/service.go in the repo

The initial .gitignore had 'service' as a bare pattern, which matches
the cmd/service directory and silently excluded the entire mock
implementation (~28KB of main source). Anchor the ignore patterns
to the repo root so they only match top-level build artefacts, and
force-add the real source now.

.gitignore

@@ -1,4 +1,4 @@
-openpayments-mock
-service
-coverage.txt
-coverage.out
+/openpayments-mock
+/service
+/coverage.txt
+/coverage.out