feature: new spells

Currently we have a couple of existing spells, but we'd love more exciting, innovative applications of spells.

Read about the [SpellsSlice](https://docs.cedarcopilot.com/spells/spells) and look at existing spells, and make a new one to add to the component registry!

Read the SpellsSlice documentation and inspect the existing spells and component registry to understand the expected shape and integration point. Propose and implement one new, innovative spell, then verify that it is registered and available alongside the existing spells.

Written by the indexing model from the issue text.