feat(agent): default to spark-2 (#4360)

Flip the `/v2/agent` request schema default from spark-1-pro to spark-2.
The controller forwards `req.body.model` verbatim upstream, so requests
that omit `model` now run spark-2.

The status controller's model fallbacks stay on spark-1-pro: they only
fire for jobs whose stored options carry no model at all, which predate
the model option and really did run spark-1-pro. Comment added so the
divergence from the new request default doesn't read as an oversight.

README's model table claimed spark-1-mini was the default, which was
already wrong; it now lists spark-2 as the default.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>