Feature: time window gating

Add a config option to `schemabot.yaml` that lets users specify when they don't want schema changes to run (e.g. market hours, Fridays, etc.). This will block schema changes from running if attempted during a window.

Start with schemabot.yaml and trace how configuration is parsed, then find the entry point that runs schema changes. Define how configured time windows are represented and evaluated, including examples such as market hours and Fridays. Done means schema changes are blocked inside a window and allowed outside it, with coverage for both cases.

Written by the indexing model from the issue text.