feat(context): create a skill writing guide

[grantcarthew]

Skills should be token efficient markdown, and do not need to spell everything out.

A skills writing guide would instruct an AI agent to intelligently write the skill so that the resulting SKILL.md file instructs the reader with the most token efficient design possible, without losing functionality.

[grantcarthew]

The document is written and stored in my private dotagents repo. Trying to decide about where to put it.