fix: reconcile run_vector_search defaults with CLI defaults

[rasros]

What

The run_vector_search signature defaults had drifted from the canonical CLI defaults in cli.py. main.py passes every argument explicitly, so the CLI path was unaffected — but any library/test caller relying on the defaults got different (and in one case broken) behavior.

Drift fixed:

Parameter	Old runner default	CLI default
pool_size	20	100
epoch_diversity	0.10	0.0
vision_model	"ensemble"	google/siglip-so400m-patch14-384
llm_rate	0.2	derived min(2/workers, 0.2)

"ensemble" was especially wrong: get_scorer forwards vision_model straight to VisionScorer as an HF repo id, so the stale default would have failed to load a model.

How

Rather than duplicate literals, the runner now imports the defaults from cli.py (the single source of truth). The import is acyclic — cli.py depends only on score/search/argparse. llm_rate defaults to None and is derived from workers via the same _default_llm_rate helper the CLI uses.

Tests

Added test_runner_defaults_match_cli_defaults, which introspects the signature and asserts each default tracks its CLI counterpart, so this can't silently drift again. Full suite: 357 passed.