feat(dev): rich mock streaming scenarios (scripted thinking + tool lifecycles)

Extracted from #39. Rewrite setupMockStreaming from a flat random-token stream into a phased state machine: thinking deltas, intro text, a successful tool call (pending, in_progress, completed, with realistic input/output/preview), middle text, a scripted failing tool call, outro, complete. Exercises the full streaming pipeline including cancellation without a goose binary.

Cleanup while porting: scenario copy in #39 embeds contributor-machine Windows paths; replace with neutral examples.

**Size**: S-M | **Depends on**: #61, #65, #66 (message contract + renderers); pairs with mock mode #58 | **Tier**: 3