Use this tool to manage the user's stored portfolio holdings.

Prefer this tool when the user wants to:
- add or update a holding
- list holdings
- remove a holding
- clear holdings

Fields for each holding:
- `ticker` (symbol)
- `price_bought` (buy price per share)
- `date_bought` (YYYY-MM-DD)

Action guide:
- `list` -> read holdings
- `upsert` -> create/update one holding
- `remove` -> delete one holding by ticker
- `clear` -> remove all holdings

After write actions (`upsert`, `remove`, `clear`), it is good practice to show the updated holdings to confirm state.
