feat: add PLAN.md and insider copytrade POC implementation

- PLAN.md: full implementation plan from issue
- config.py: configurable thresholds, API keys via .env
- ingestion/: EDGAR RSS poller + Form 4 XML parser
- db/: SQLite schema + interface (WAL mode)
- signals/: filter engine (buy/10b5-1/value/role) + cluster detector
- alerts/: Slack webhook alert with score gating
- broker/: Alpaca paper/live trade execution
- backtest/: historical signal backtesting with yfinance
- main.py: CLI entrypoint (run | fetch-once | backtest)

.env.example

@@ -0,0 +1,6 @@
+SLACK_WEBHOOK_URL=
+ALPACA_KEY=
+ALPACA_SECRET=
+ALPACA_BASE_URL=https://paper-api.alpaca.markets
+DB_PATH=insider.db
+DATA_DIR=data/filings