new skill to modify the schema

when someone wants to change/add a column, change/add a table, etc. We should have a skill dedicated to this which talks about Felis and also updating tests. Encourages user to think about adding unit name to column name if not adding a unit column. May also give some advice on if this change is a good idea or if there's a different/better way to do whatever the user is trying to do.

No file or test path is named. Start by locating the existing skill entry points and their tests, then review how Felis is currently referenced. Done means the new skill covers schema changes, test updates, unit naming, and considering alternatives, with corresponding tests.

Written by the indexing model from the issue text.