feat(dev): explicit mock mode via goose.binaryPath = "mock"

Extracted from #39. Today mock streaming is only an implicit fallback when spawn/handshake fails. Setting `goose.binaryPath` to `mock` should explicitly short-circuit activation into mock streaming before binary discovery.

Constraints: the short-circuit must live inside the single-flight bootstrap from #51, bypass the loud invalid-path failure from #50, and behave sanely on restart into/out of mock. Document the mode in DEVELOPMENT.md (currently undocumented).

**Size**: S | **Depends on**: nothing | **Tier**: 0 (quick win)