fix: market_context must not import runtime (research-isolation guard)

The DB-config wiring was unnecessary there anyway — market_context reads
zero trading knobs; the guard test (plan 009) rightly rejected the import.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

research/ingestion/market_context.py

@@ -26,12 +26,6 @@ from research.ingestion.market_data import (  # noqa: E402
     upsert_daily_bars, upsert_minute_bars)
 from research.multi_schema import open_research_db  # noqa: E402
 
-# config parity with the live loop: DB-backed knobs (config_overrides) beat
-# compose env — a bare env would replay under DEFAULT exits (the 4.5x-baseline
-# landmine, orderflow-shelf-verdicts 2026-07-17)
-from runtime.env_overrides import load as _load_cfg_overrides  # noqa: E402
-_load_cfg_overrides()
-
 INDEX_SYMBOLS = ("SPY", "QQQ")