Make DatabaseTool use a read-only transaction

One cool feature I have seem from some other Agent database tools is to use enclose all db operations in a READ ONLY transaction as a protection mechanism. I think that is a good idea. We should add that to https://github.com/supercog-ai/agentic/blob/main/src/agentic/tools/database_tool.py Some notes: - Someone might want to use the DB tool for mutations, so we should add an optional flag like "allow_mutations=False" to disable the read only (but that should be the default). - Postgres syntax: https://www.postgresql.org/docs/current/sql-set-transaction.html - There is probably NOT any database-agnostic syntax for doing this. This might be a good job for an LLM - to enumerate a big switch statement that returns the right syntax based on the database type. Otherwise we could JUST implement support for postgres/mysql/sqlite. - Another idea is if we don't know the db type, we could just force a ROLLBACK after the query. I doubt this will work for 100% of the cases (eg if you DROP TABLE does the rollback help?) Again writing tests for this will be a pain to cover the different db types.