skill-quality: add completion-criteria check criterion + pointer to write-for-agents doctrine

[kyle-sexton]

Context

Second build deliverable of course lane 7 (#2909; design contract docs/specs/write-for-agents-brief.md, Q5: completion-criteria doctrine adopted at BOTH moments). The write-side lands in #2962 (docs-hygiene:write-for-agents); this issue is the audit-side, so the 200+ already-written skills get graded on the doctrine too, and skill authors reach it from the skill-quality surface.

Proposed work

• Add a completion-criteria criterion to skill-quality:check: steps state completion criteria with clarity AND demand; no premature-completion shapes (a step satisfiable before its goal-state); post-completion obligations stated; agent does the legwork (steps don't offload resolvable facts to the human)
• Add a pointer from the skill-quality surface to the docs-hygiene:write-for-agents doctrine so SKILL.md authors reach it despite the route-away trigger fence (design decision Q2)
• skill-quality version bump + CHANGELOG entry

Acceptance criteria

• skill-quality:check grades completion criteria (criterion visible in its check set; settled by the diff)
• Pointer to the write-for-agents doctrine present on the skill-quality surface (settled by the diff)
• Sequencing: lands with or after docs-hygiene: implement write-for-agents — authoring-time skill for agent-consumed markdown #2962 (the pointer needs a target)

Metadata

Field	Value
Category	unspecified
Area	skill-quality
Ecosystem	unspecified