Add a Vanguard transaction-history adapter

[DanMat]

Goal

Same as the Schwab adapter, for Vanguard exports. The engine needs a contribution calendar (date, contributed); optionally dividends received (date, symbol, dividend).

What to do

1. Copy src/divvy/importers/broker_template.py to src/divvy/importers/vanguard.py.
2. Parse Vanguard's transaction CSV into the documented schemas — see src/divvy/ledger.py (Fidelity) as the reference.
3. Add a test with a small fake fixture (follow tests/test_ledger.py). No real statements.

Acceptance criteria

• contributions() returns date, contributed; test passes; ruff clean.

See CONTRIBUTING.md. Pairs well with the Schwab adapter issue.