skills: tell the whole story in the public skill, and fix what went stale (#100)

The skill described the mechanics of the platform without the problem it
solves, so a coding agent reading it could operate Ellipsis but not say why
a team would adopt it. Adds the fleet-management problem, why cloud beats
laptop, and the benefits that were missing: usage-based pricing, any model,
structured output as a contract, and session observability.

Code review had drifted furthest from the product. include_default_reviewers
is gone, position now decides what a code_review.yaml governs, and
enabled: false does the opposite of what the skill claimed. Also corrects
'agent session handoff', which was shown without its required --parent.

Author: hbrooks