Add skill-authoring-gate Chain Skill

[Dimon94]

Parent

#40

What to build

Add a Chain Skill that defines the quality gate for writing and editing cc-devflow skills. It should be reusable by later skill-contract work and should turn the writing-great-skills principles into this repo's own skill authoring contract.

The slice is complete when a maintainer can point a skill change at this gate and get a clear answer on invocation, information hierarchy, completion criteria, progressive disclosure, leading words, and pruning.

Acceptance criteria

• A Chain Skill exists for skill authoring quality and is not treated as a human workflow entry point.
• The gate covers invocation class, information hierarchy, completion criteria, progressive disclosure, leading words, and pruning.
• The gate distinguishes User Entry Skill, Chain Skill, and Agent-facing internal contract language.
• At least one focused test or contract check proves the authoring gate is available for other skills to read.
• Existing public workflow behavior is unchanged.

Blocked by

None - can start immediately