signal_log: persist agent panel verdict + per-persona takes to SQLite

Add panel_verdict, panel_confidence, panel_rationale, panel_takes_json
columns to the signals table (additive, idempotent ALTER TABLE migration
for existing DBs). _report_row now reads the panel fields attach_panel
writes onto the SignalReport, so LLM panel verdicts stop evaporating on
process exit. Also documents the Oanda re-pin (round_trip_cost stays
$0.30 for cross-book comparability and the live TP gate; $0.65 is the
measured headline) and adds the feed-divergence / parity / shadow-scan
study tooling referenced by CLAUDE.md.

providers/tiingo.py

@@ -3,6 +3,8 @@ ForexBot2 — Tiingo data provider
 =================================
 OHLCV candles from Tiingo's FX endpoint
 (`/tiingo/fx/{ticker}/prices`, resampled mid-price bars, no volume).
+Tiingo is streaming-only in production: its REST candles remain available
+for comparison research, but it is not a designated candle fallback.
 All shared behavior (cache, validation, boundary guarantees, range
 pagination) lives in `BaseDataProvider`; this module only maps
 symbols/resample frequencies, sizes the request window, and parses