- budget/budget_test.go: add TestFit_DesignDocsInSystemPrompt,
TestFit_DesignDocsTrimmedBeforeFileContext, TestFit_DesignDocsEmptyNoHeading
to cover the new DesignDocs section through Fit() and buildResult()
- Remove PLAN-137.md (contained raw thinking stream, not suitable as repo doc)
- Add docs/DESIGN-137-doc-map.md with clean architectural decision record