VS3: cursor-cloud model resolution + /model override

[Praneeth16]

Parent

omnigent-ai#1148

What to build

Flow model selection through Omnigent's per-harness model-env + /model override path for cursor-cloud, with a cloud-appropriate default when unset. Drop a databricks-* id authored for another harness.

Acceptance criteria

• _HARNESS_MODEL_ENV_KEY maps cursor-cloud → HARNESS_CURSOR_CLOUD_MODEL
• cursor-cloud added to model_override._SDK_MODEL_OVERRIDE_HARNESSES
• cloud default model applied when unset; databricks-* dropped
• /model override round-trips into the child env
• unit tests

Blocked by

• VS1: cursor-cloud harness selectable end-to-end (tracer bullet) #1