Cleanup: remove dead code, stale docs, and all non-main branches

- Delete back-compat shims data_provider.py / oanda_provider.py;
  test_all.py now imports providers.* directly (ForexDataProvider name
  retired; the two shim-only tests removed, 268 -> 266 tests)
- Delete backtest/research/calendar_store.py (no importer, no entry
  point); config.py audit comment reworded
- Delete executed plan doc (2026-07-21 centralized-config) and the two
  unbuildable guide PDFs; HTML guides remain the source of truth
- OANDA_MIGRATION_ROADMAP.md status header corrected: phases 1-2
  shipped 2026-07-28, feed is live on Oanda; phases 3-6 still unbuilt
- Branches deleted locally + on both remotes: oanda_demo (merged),
  web-dashboard, eurusd-modular, reversal-exit-alert (owner-approved
  permanent deletion); July stash dropped

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_019zR58p8ACW1oJ5QXVUZ5es

config.py

@@ -146,7 +146,7 @@ class NewsConfig:
     # against the validated 12mo XAU M5 book (547 trades, +139.9R)
     # showed they suppress 25-35 trades worth +4.6..+7.2R/yr — trades
     # entered near USD releases OUTPERFORM (+0.475R vs +0.256R avg).
-    # Audit 2026-07-11 via backtest/research/calendar_store.
+    # Audit 2026-07-11 (calendar-replay study; tooling since removed).
     event_risk_caps_signals: bool = False
     # Whether headline sentiment moves the SIGNAL (aligned: +5 confidence;
     # conflicting: 10-point penalty). Warnings are always emitted