Add a Charles Schwab transaction-history adapter

[DanMat]

Goal

Let Schwab users bring their own data. The backtest engine only needs a contribution calendar (date, contributed); optionally also dividends received (date, symbol, dividend) for the real-account comparison row.

What to do

1. Copy src/divvy/importers/broker_template.py to src/divvy/importers/schwab.py.
2. Implement contributions() (and optionally dividends_actual()) to parse Schwab's Transactions CSV export into the documented schemas. See src/divvy/ledger.py for the complete Fidelity example (how it classifies buys vs. reinvestments vs. dividends).
3. Add a test with a small fake fixture (follow tests/test_ledger.py). Never commit a real statement.
4. Optionally wire a --broker schwab path into divvy compare in src/divvy/cli.py.

Acceptance criteria

• contributions() returns columns date, contributed (positive $, DRIP reinvestments excluded).
• Test passes; ruff check + ruff format clean.

Good first issue — self-contained, one new file + one test. See CONTRIBUTING.md.