The MCP server should be able to take a sqlite database command line argument

If you invoke it as
```sh
python -m typeagent.mcp.server -d foo.db
```
It should take an existing foo.db database and query against that (and not try to fill it with the podcast transcript from testdata).

Two additional refactors:

- Use the high-level APIs
- Add a new tool to add one or more messages

(Might also come up with a config file?)
