Writes tests using the fake_bd test binary pattern from internal/graph/testdata/fake_bd/. Covers building fake binary, setting FAKE_BD_* env vars for canned responses, capture files for arg/env verification, and serverState/hookState setup. Use when user says 'write tests', 'add test', 'test coverage', or needs to test graph/mcp/hook code that calls bd. Do NOT use for tests that don't invoke the bd CLI binary (e.g., pure utility functions, config parsing, CLI flag tests).