feat: add kill-switch control flow

trex_core/adapters/persistence/sql/005_create_killswitch_exclusion.sql

@@ -0,0 +1,15 @@
+-- BAT-012 / REQ-044: day-scoped kill-switch exclusion overlay.
+-- Distinct from snapshot_pause (permanent, resume-until). A row excludes a
+-- portfolio (or '*' for global) from strategy execution for ONE trading_date;
+-- the next trading day is automatically clear because reads filter by
+-- trading_date. Manual resume (REQ-046) deletes the row.
+CREATE TABLE IF NOT EXISTS killswitch_exclusion (
+    portfolio_id  TEXT NOT NULL,   -- '*' = global exclusion sentinel
+    trading_date  DATE NOT NULL,
+    actor         TEXT,
+    created_at    TIMESTAMPTZ NOT NULL DEFAULT (now() AT TIME ZONE 'UTC'),
+    PRIMARY KEY (portfolio_id, trading_date)
+);
+
+CREATE INDEX IF NOT EXISTS idx_killswitch_exclusion_trading_date
+    ON killswitch_exclusion (trading_date);