fix(test): add opencode stub binary for agent config tests

TestRoleAgentConfigWithCustomAgent tests custom agent "opencode-mayor"
which uses the opencode binary. Without the stub, the test falls back
to default and fails assertions about prompt_mode and env vars.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/config/test_main_test.go

@@ -22,6 +22,7 @@ func TestMain(m *testing.M) {
 		"cursor-agent",
 		"auggie",
 		"amp",
+		"opencode",
 	}
 	for _, name := range binaries {
 		path := filepath.Join(stubDir, name)