Improve fundamental skill about references to system tables and how to look up table names

> Wonderful, TY for writing this together. IMHO it makes sense to add references to the sys tables (I have a particular ops approach) and also hint how to lookup table, names and explain the usage of "schema".

_Originally posted by @WalBeh in https://github.com/crate/agent-skills/pull/1#pullrequestreview-4225926585_

Start by locating the skill document covered by the original pull request and review its current guidance on tables and schema. Update the document with references to system tables, instructions for looking up table names, and an explanation of schema usage; done means all three topics are addressed clearly.

Written by the indexing model from the issue text.