test: add default git_orgs fallback render coverage

[tsavo-at-pieces]

Context:\nWorkflow generator supports configurable ci.git_orgs with default fallback to open-runtime and pieces-app when omitted.\n\nGap:\n- We have tests for custom git_orgs, but not an explicit render assertion for default fallback behavior when ci.git_orgs is absent.\n\nProposed scope:\n- Add regression test verifying default org rewrite rules are rendered when ci.git_orgs is not set.\n\nReference:\n- Merged baseline changes: #36