Adds a Gemini CLI custom command (`/explain`) to generate skills for
working with code packages in this repo. Usage:
1. Add skill for a new package:
`/explain packages/common`
2. Update the skill for a package:
`/explain Update the skill for packages/core`
3. Add supplemental information:
`/explain Add supplemental info to the signal forms skill about how
schema logic is implemented`
Also adds a few skills for working with some of the packages I commonly
interact with, including:
- packages/forms/signals
- packages/core
- packages/compiler-cli
Andrew Scott